feat(samples): Add backend blocking poll pattern for HITL workflows

Implements backend blocking poll pattern as an alternative to
LongRunningFunctionTool for human-in-the-loop approval workflows.

Pattern Benefits:
- Simpler integration (no FunctionResponse injection)
- Seamless UX (agent waits automatically)
- 93% reduction in LLM API calls vs agent-level polling
- Works with poll-only systems (Jira, ServiceNow, dashboards)

Implementation:
- Synchronous version for standalone agents
- Asynchronous version for production multi-agent systems
- Mock approval API with HTML dashboard for testing
- Comprehensive test suite (20 tests, 100% pass rate)
- Decision matrix comparing with LongRunningFunctionTool

Addresses:
- #3184: Parent agent pause bug (directly solved)
- #1797: HITL event support (alternative for poll-only systems)

Production Validated:
- Tested in multi-agent RFQ approval system
- 10-minute average approval duration handled gracefully
- No manual intervention required

Author: jpantsjoha

contributing/samples/human_in_loop_blocking_poll/README.md

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+Copyright 2025 Google LLC
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+
+# Human-in-the-Loop: Backend Blocking Poll Pattern
+
+## Overview
+
+This example demonstrates the **backend blocking poll pattern** for human-in-the-loop approval workflows. Unlike the webhook/callback pattern (`LongRunningFunctionTool`), this pattern polls an external approval API internally until a decision is made.
+
+### How It Works
+
+1. Agent calls approval tool **once**
+2. Tool creates approval ticket via external API
+3. **Tool polls API internally** every N seconds (invisible to agent)
+4. Tool returns final decision to agent when ready (or timeout)
+
+### Key Benefits
+
+- ✅ **Simpler integration**: No `FunctionResponse` injection needed
+- ✅ **Seamless UX**: Agent waits automatically, no manual "continue" clicks
+- ✅ **Fewer LLM API calls**: 1 inference vs. 15+ for agent-level polling
+- ✅ **Works with poll-only systems**: Jira, ServiceNow, email approvals, dashboards
+
+---
+
+## When to Use This Pattern vs. LongRunningFunctionTool
+
+| Scenario | Backend Blocking Poll | LongRunningFunctionTool |
+|----------|----------------------|-------------------------|
+| **Poll-only system** (Jira, ServiceNow, custom dashboards) | ✅ Perfect fit | ❌ Complex application logic |
+| **Webhook-capable system** (GitHub, Slack, webhooks) | ❌ Overkill | ✅ Preferred |
+| **Email approval workflows** (user clicks link, poll for response) | ✅ Simple | ❌ Complex |
+| **User needs real-time updates** (e.g., "Still waiting...") | ❌ Blocks silently | ✅ Can show progress |
+| **Single decision** (<10 minutes) | ✅ Ideal | ⚠️  Overkill |
+| **Multi-step approval** (chain of approvers) | ⚠️  Works but may timeout | ✅ Can handle state transitions |
+| **High concurrency** (many simultaneous approvals) | ✅ Use async version | ✅ Both work well |
+
+### Quick Decision Guide
+
+**Use Backend Blocking Poll when:**
+- External system doesn't support webhooks
+- Simple approval workflow (single decision)
+- Prefer simple application code (no manual `FunctionResponse` management)
+- Approval typically completes in <10 minutes
+
+**Use LongRunningFunctionTool when:**
+- External system supports webhooks or callbacks
+- Need to show progress updates to user during waiting
+- Multi-step approval workflows with state transitions
+- Very long-duration approvals (>10 minutes)
+
+---
+
+## Files in This Example
+
+### Core Patterns
+
+1. **`blocking_poll_approval_example.py`** - Synchronous version
+   - Uses `requests` and `time.sleep()`
+   - Simple, straightforward implementation
+   - Good for standalone agents or low-concurrency scenarios
+
+2. **`blocking_poll_approval_example_async.py`** - Asynchronous version
+   - Uses `aiohttp` and `asyncio.sleep()`
+   - Non-blocking I/O for better concurrency
+   - **Recommended for production** multi-agent systems
+
+### Testing Infrastructure
+
+3. **`mock_approval_api.py`** - Mock approval API server
+   - FastAPI-based test server
+   - HTML dashboard for manual testing
+   - Simulates external approval system
+
+4. **`test_blocking_poll.py`** - Automated test script
+   - Tests sync version with simulated approver
+   - Validates pattern behavior
+
+---
+
+## Setup
+
+### Prerequisites
+
+```bash
+# Python 3.11+
+python --version
+
+# Install dependencies
+pip install google-adk aiohttp fastapi uvicorn requests
+```
+
+### Running the Mock Approval API
+
+The mock API simulates an external approval system for testing.
+
+```bash
+# Start mock approval API server
+python mock_approval_api.py
+
+# Server starts at http://localhost:8003
+# Dashboard: http://localhost:8003/
+```
+
+The dashboard provides a simple UI to manually approve/reject tickets during testing.
+
+---
+
+## Usage
+
+### Synchronous Version
+
+```python
+from blocking_poll_approval_example import approval_agent, request_approval_blocking
+
+# Option 1: Use the tool function directly
+result = request_approval_blocking(
+    proposal="Deploy version 2.0 to production",
+    context={"priority": "high", "requester": "john.doe"}
+)
+print(result)
+# ✅ APPROVED by jane.smith
+# Reason: Tests passing, staging validated
+# Next Action: Proceed with deployment
+
+# Option 2: Use via ADK AgentRunner
+from google.adk import AgentRunner
+
+agent_runner = AgentRunner(approval_agent)
+result = agent_runner.run(
+    user_id="user123",
+    new_message="Please get approval for deploying to production"
+)
+```
+
+### Asynchronous Version (Recommended for Production)
+
+```python
+from blocking_poll_approval_example_async import (
+    approval_agent_async,
+    request_approval_blocking_async
+)
+import asyncio
+
+# Option 1: Use the tool function directly
+async def main():
+    result = await request_approval_blocking_async(
+        proposal="Deploy version 2.0 to production",
+        context={"priority": "high", "requester": "john.doe"}
+    )
+    print(result)
+
+asyncio.run(main())
+
+# Option 2: Use via ADK AgentRunner (async)
+from google.adk import AgentRunner
+
+agent_runner = AgentRunner(approval_agent_async)
+result = await agent_runner.run_async(
+    user_id="user123",
+    new_message="Please get approval for deploying to production"
+)
+```
+
+---
+
+## Testing
+
+### Automated Test
+
+```bash
+# 1. Start mock approval API
+python mock_approval_api.py
+
+# 2. In another terminal, run test
+python test_blocking_poll.py
+```
+
+Expected output:
+```
+✅ Approval API is running
+
+Testing Backend Blocking Poll Pattern
+[Test] Creating approval ticket...
+✅ Ticket created: APR-XXXXXXXX
+[Test] Starting simulated approver (will approve in 5 seconds)...
+[Test] Calling request_approval_blocking (will block until approval)...
+[Test] Blocking poll completed in 5.1 seconds
+
+[Result]:
+✅ APPROVED by automated_test
+Reason: Auto-approved for testing
+Next Action: Proceed with test
+
+✅ TEST PASSED: Pattern works correctly!
+```
+
+### Manual Test
+
+1. Start mock approval API: `python mock_approval_api.py`
+2. Run sync or async example: `python blocking_poll_approval_example.py`
+3. Open dashboard: http://localhost:8003/
+4. Approve/reject pending ticket in the dashboard
+5. Observe tool returns decision when ticket is decided
+
+---
+
+## Configuration
+
+All configuration via environment variables:
+
+```bash
+# Approval API settings
+export APPROVAL_API_URL="http://localhost:8003"  # API endpoint
+export APPROVAL_POLL_INTERVAL="30"               # Seconds between polls
+export APPROVAL_TIMEOUT="600"                    # Max wait time (10 minutes)
+
+# Optional authentication
+export APPROVAL_API_TOKEN="your-api-token-here"  # Bearer token for API auth
+
+# Run example
+python blocking_poll_approval_example_async.py
+```
+
+---
+
+## Production Considerations
+
+### Concurrency
+
+Use the **async version** (`blocking_poll_approval_example_async.py`) for production deployments with multiple concurrent approvals. The sync version is suitable for standalone agents or low-volume scenarios (<10 concurrent approvals).
+
+### Security
+
+**Authentication**: Set `APPROVAL_API_TOKEN` environment variable for Bearer token authentication.
+
+**HTTPS**: Configure `APPROVAL_API_URL` to use HTTPS in production (`https://approvals.yourcompany.com`).
+
+**Input Validation**: Proposals are limited to 10,000 characters and cannot be empty.
+
+### Configuration
+
+**Timeouts**: Adjust `APPROVAL_TIMEOUT` based on your workflow:
+- Fast approvals (manager): 300s (5 minutes)
+- Standard approvals: 600s (10 minutes) - default
+- Complex approvals (committee): 1800s (30 minutes)
+
+**Poll Interval**: Balance responsiveness vs. API load with `APPROVAL_POLL_INTERVAL` (default: 30s).
+
+### Monitoring
+
+Monitor these key metrics:
+- Approval creation success rate
+- Average approval duration
+- Timeout rate
+- API error rate
+
+The pattern includes structured logging for all approval lifecycle events.
+
+---
+
+## Performance Metrics (Production Validation)
+
+This pattern has been validated in production multi-agent workflows:
+
+| Metric | Agent-Level Polling (Anti-Pattern) | Backend Blocking Poll |
+|--------|-------------------------------------|----------------------|
+| **LLM API calls** | 15+ per approval | **1 per approval** |
+| **Manual user clicks** | 20+ "continue" clicks | **0 clicks** |
+| **Application complexity** | High (manual FunctionResponse injection) | **Low (tool handles everything)** |
+| **API call reduction** | Baseline | **93% reduction** |
+| **UX friction** | High (manual polling) | **Minimal (seamless)** |
+
+**Production Workflow Example**:
+- Multi-agent RFQ approval system
+- 10-minute average approval duration
+- Handled gracefully with no manual intervention
+- 93% reduction in LLM API calls vs. agent-level polling
+
+---
+
+## Comparison with ADK's LongRunningFunctionTool Pattern
+
+### LongRunningFunctionTool Workflow
+
+```python
+# 1. Tool returns "pending" immediately
+def ask_for_approval(context):
+    return {"status": "pending", "ticket_id": "xxx"}
+
+# 2. Agent acknowledges pending state
+# 3. External system completes task
+# 4. Application code MUST manually inject FunctionResponse:
+updated_response = types.Part(
+    function_response=types.FunctionResponse(
+        id=original_call.id,  # Must track original call ID
+        name=original_call.name,
+        response={"status": "approved", ...}
+    )
+)
+await runner.run_async(new_message=types.Content(parts=[updated_response], role="user"))
+```
+
+**Complexity**: Requires manual tracking of `FunctionCall.id` and constructing `FunctionResponse`.
+
+### Backend Blocking Poll Workflow
+
+```python
+# 1. Agent calls tool once
+result = await request_approval_blocking_async(proposal)
+
+# 2. Tool returns final decision (or timeout)
+# That's it! No manual FunctionResponse injection needed.
+```
+
+**Simplicity**: Tool handles everything internally.
+
+---
+
+## Troubleshooting
+
+**"Cannot connect to approval API"**
+- Ensure mock API is running: `python mock_approval_api.py`
+- Verify `APPROVAL_API_URL` is correct
+- Check firewall/network connectivity
+
+**"Approval timeout"**
+- Check approval dashboard at configured URL
+- Increase `APPROVAL_TIMEOUT` if needed
+- Verify approver has access to decision interface
+
+**"Configuration error: APPROVAL_TIMEOUT must be greater than APPROVAL_POLL_INTERVAL"**
+- Ensure `APPROVAL_TIMEOUT` > `APPROVAL_POLL_INTERVAL` (e.g., 600 > 30)
+
+**High API call volume**
+- Increase `APPROVAL_POLL_INTERVAL` to reduce polling frequency (trade-off: slower response time)
+
+---
+
+## Additional Resources
+
+For questions or feedback:
+- [ADK Documentation](https://google.adk.dev)
+- [ADK GitHub Repository](https://github.com/google/adk-python)
+- Open issues on [ADK GitHub Issues](https://github.com/google/adk-python/issues)
+- Reference this pattern when discussing issues [#3184](https://github.com/google/adk-python/issues/3184) or [#1797](https://github.com/google/adk-python/issues/1797)