test(hardware): add comprehensive error path tests for hardware graph

[jra3]

Summary

This PR adds comprehensive error path testing for hardware graph components to achieve >80% test coverage for previously under-tested error scenarios, addressing issue #160.

The implementation follows a three-phase testing approach covering critical error paths, data validation edge cases, and recovery scenarios.

What Changed

New Test File

• internal/hardware/graph/error_paths_test.go (812 lines)
  • MockStore implementation for controlled error injection testing
  • 15 comprehensive test functions covering all three testing phases
  • Systematic coverage of error scenarios across hardware graph operations

Dependencies

• Added github.com/stretchr/objx v0.5.2 (required by testify/mock)

Test Coverage by Phase

Phase 1: Critical Error Paths

• File System Access (getOSInfo): Missing files, permission denied, malformed content, empty files, quote handling
• System Information (getSystemInfo): Graceful error handling with fallback values for uname failures
• Resource Store Failures: System node creation, CPU package storage, CPU core storage, and relationship persistence errors

Phase 2: Data Validation

• Network Interface Enums: Unknown duplex modes, interface types, operational states, empty values, case variations
• Bus Connection Enums: All bus types (PCI, PCIe, NVMe, SATA, USB, I2C, SPI, etc.) including unknown value handling
• Protobuf Marshaling: Nil pointer handling and error propagation through relationship creation
• Containment Types: Physical, logical, partition mappings with proper unknown value handling

Phase 3: Recovery Testing

• Partial Failures: Missing physical IDs for CPU cores, duplicate processor IDs in CPU info
• NUMA Topology Errors: Store failures during node creation, invalid node distance references
• Disk Topology Errors: Partition storage failures with proper error propagation
• Duplicate Handling: Graceful skipping of existing relationships, proper query error handling
• Edge Cases: Bus type inference for all device name patterns, nil snapshot component handling

Test Results

Coverage Improvement:

• Before: 70.9%
• After: 80.4%
• Improvement: +9.5 percentage points ✅

All tests passing:

ok  	github.com/antimetal/agent/internal/hardware/graph	32.709s	coverage: 80.4% of statements

Success Criteria Met

✅ Increase error path coverage to >80% for identified functions
✅ Test all identified failure scenarios from issue analysis
✅ Validate proper error messages and context propagation
✅ Ensure no panics on invalid input or edge cases

Testing Instructions

# Run all hardware graph tests
go test ./internal/hardware/graph/...

# Run with coverage report
go test ./internal/hardware/graph/... -cover

# Run specific error path tests
go test ./internal/hardware/graph/... -run "Error|EdgeCase|Recovery"

# Generate detailed coverage profile
go test ./internal/hardware/graph/... -coverprofile=coverage.out
go tool cover -html=coverage.out

Implementation Details

MockStore Pattern

Uses testify/mock to create a controllable mock of the resource store interface, enabling:

• Precise error injection at specific operations
• Verification of call patterns and arguments
• Simulation of race conditions and partial failures

Error Injection Strategy

Each test systematically exercises error paths:

1. Setup mock to return specific error at exact operation
2. Execute function under test
3. Verify error is properly propagated with context
4. Confirm no panic or undefined behavior

Edge Case Coverage

Tests handle realistic failure scenarios:

• Filesystem errors (ENOENT, EACCES)
• Malformed data files (invalid JSON, missing fields)
• Concurrent access patterns
• Resource exhaustion situations

Closes #160