fix: map A2A Message.role to correct GenAI content role

Merge #5198

## Summary

Fixes #5186 — `convert_a2a_message_to_event()` hard-codes `role="model"` for all resulting ADK events, regardless of the original A2A `Message.role`. User messages (`Role.user`) are incorrectly restored as model events.

**Root cause:** `_create_event()` always creates `genai_types.Content(role="model", ...)` without considering the source message's role.

**Fix:**
- Add `_a2a_role_to_content_role()` helper that maps `A2ARole.user` → `"user"` and `A2ARole.agent` → `"model"`
- Add `content_role` parameter to `_create_event()` (defaults to `"model"` for backward compatibility)
- Pass the mapped role from `convert_a2a_message_to_event()` through to `_create_event()`

## Changes
- `src/google/adk/a2a/converters/to_adk_event.py`: Import `Role`, add role mapping helper, thread `content_role` through `_create_event()`
- `tests/unittests/a2a/converters/test_to_adk.py`: Add 2 tests verifying user→"user" and agent→"model" role mapping

## Test plan
- [x] All 16 `test_to_adk.py` tests pass (14 existing + 2 new)
- [x] New tests verify: `Role.user` → `content.role == "user"`, `Role.agent` → `content.role == "model"`
- [x] No regressions in existing converter tests

Co-authored-by: Haran Rajkumar <haranrk@google.com>
COPYBARA_INTEGRATE_REVIEW=#5198 from enjoykumawat:fix/a2a-message-role-mapping da6826d
PiperOrigin-RevId: 933926812

Author: 2 people

src/google/adk/a2a/converters/to_adk_event.py

@@ -24,6 +24,7 @@
 
 from a2a.types import Message
 from a2a.types import Part as A2APart
+from a2a.types import Role as A2ARole
 from a2a.types import Task
 from a2a.types import TaskArtifactUpdateEvent
 from a2a.types import TaskState
@@ -187,6 +188,7 @@ def _create_event(
     actions: Optional[EventActions] = None,
     long_running_function_ids: Optional[set[str]] = None,
     partial: bool = False,
+    content_role: str = "model",
 ) -> Optional[Event]:
   """Creates an ADK event from parts and metadata."""
   event_actions = actions or EventActions()
@@ -209,7 +211,7 @@ def _create_event(
       ),
       content=(
           genai_types.Content(
-              role="model",
+              role=content_role,
               parts=output_parts,
           )
           if output_parts
@@ -221,6 +223,13 @@ def _create_event(
   return event
 
 
+def _a2a_role_to_content_role(role: Optional[A2ARole]) -> str:
+  """Maps an A2A Role to the corresponding GenAI content role."""
+  if role == A2ARole.user:
+    return "user"
+  return "model"
+
+
 def _parse_adk_metadata_value(value: Any) -> Any:
   """Parses ADK metadata values serialized through A2A."""
   if not isinstance(value, str):
@@ -468,11 +477,13 @@ def convert_a2a_message_to_event(
     output_parts, _ = _convert_a2a_parts_to_adk_parts(
         a2a_message.parts, part_converter
     )
+    content_role = _a2a_role_to_content_role(getattr(a2a_message, "role", None))
     return _create_event(
         output_parts,
         invocation_context,
         author,
         _extract_event_actions(a2a_message.metadata),
+        content_role=content_role,
     )
 
   except Exception as e:

tests/unittests/a2a/converters/test_to_adk.py

@@ -20,6 +20,7 @@
 from a2a.types import Artifact
 from a2a.types import Message
 from a2a.types import Part as A2APart
+from a2a.types import Role as A2ARole
 from a2a.types import Task
 from a2a.types import TaskArtifactUpdateEvent
 from a2a.types import TaskState
@@ -643,3 +644,41 @@ def test_convert_a2a_artifact_update_to_event_none(self):
     """Test convert_a2a_artifact_update_to_event with None."""
     with pytest.raises(ValueError, match="A2A artifact update cannot be None"):
       convert_a2a_artifact_update_to_event(None)
+
+  def test_convert_a2a_message_to_event_user_role(self) -> None:
+    """Test that A2A user role maps to GenAI content role 'user'."""
+    a2a_part = Mock(spec=A2APart)
+    a2a_part.root = Mock(spec=TextPart)
+    a2a_part.root.metadata = {}
+    message = Message(message_id="msg-1", role=A2ARole.user, parts=[a2a_part])
+
+    mock_genai_part = genai_types.Part.from_text(text="hello from user")
+    mock_part_converter = Mock(return_value=[mock_genai_part])
+
+    event = convert_a2a_message_to_event(
+        message,
+        author="user",
+        invocation_context=self.mock_context,
+        part_converter=mock_part_converter,
+    )
+
+    assert event.content.role == "user"
+
+  def test_convert_a2a_message_to_event_agent_role(self) -> None:
+    """Test that A2A agent role maps to GenAI content role 'model'."""
+    a2a_part = Mock(spec=A2APart)
+    a2a_part.root = Mock(spec=TextPart)
+    a2a_part.root.metadata = {}
+    message = Message(message_id="msg-1", role=A2ARole.agent, parts=[a2a_part])
+
+    mock_genai_part = genai_types.Part.from_text(text="hello from agent")
+    mock_part_converter = Mock(return_value=[mock_genai_part])
+
+    event = convert_a2a_message_to_event(
+        message,
+        author="test-agent",
+        invocation_context=self.mock_context,
+        part_converter=mock_part_converter,
+    )
+
+    assert event.content.role == "model"