fix: guard against None converter results in RemoteA2aAgent

Merge #5199

## Summary

Fixes #5187

- **Problem:** `RemoteA2aAgent._handle_a2a_response` and `_handle_a2a_response_v2` crash with `AttributeError` when converter functions (`convert_a2a_task_to_event`, `convert_a2a_message_to_event`, or config-based converters) return `None`. The code immediately accesses `.custom_metadata` on the result without checking for `None`.
- **Fix:** Added `None`-checks after every converter call in both handler methods. When a converter returns `None`, the handler now gracefully returns `None` (skip the event) instead of crashing.
- **Tests:** Added `TestHandleNoneConverterResults` with three test cases covering the `None` return path for message converters and task converters in both v1 and v2 handlers.

## Test plan

- [x] `python -m pytest tests/unittests/agents/test_remote_a2a_agent.py -v` -- all 98 tests pass
- [x] New tests specifically verify that `_handle_a2a_response` returns `None` when `convert_a2a_message_to_event` returns `None`
- [x] New tests verify that `_handle_a2a_response` returns `None` when `convert_a2a_task_to_event` returns `None`
- [x] New tests verify that `_handle_a2a_response_v2` returns `None` when `a2a_message_converter` returns `None`

Co-authored-by: George Weale <gweale@google.com>
COPYBARA_INTEGRATE_REVIEW=#5199 from enjoykumawat:fix/remote-a2a-agent-none-check 8550233
PiperOrigin-RevId: 933978968

Author: 2 people

src/google/adk/agents/remote_a2a_agent.py

@@ -533,6 +533,8 @@ async def _handle_a2a_response(
           event = convert_a2a_message_to_event(
               update.status.message, self.name, ctx, self._a2a_part_converter
           )
+          if not event:
+            return None
           if event.content is not None and update.status.state in (
               TaskState.submitted,
               TaskState.working,
@@ -559,6 +561,8 @@ async def _handle_a2a_response(
           # for now.
           return None
 
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
         event.custom_metadata[A2A_METADATA_PREFIX + "task_id"] = task.id
         if task.context_id:
@@ -571,6 +575,8 @@ async def _handle_a2a_response(
         event = convert_a2a_message_to_event(
             a2a_response, self.name, ctx, self._a2a_part_converter
         )
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
 
         if a2a_response.context_id:
@@ -641,6 +647,8 @@ async def _handle_a2a_response_v2(
         event = self._config.a2a_message_converter(
             a2a_response, self.name, ctx, self._config.a2a_part_converter
         )
+        if not event:
+          return None
         event.custom_metadata = event.custom_metadata or {}
 
         if a2a_response.context_id:

tests/unittests/agents/test_remote_a2a_agent.py

@@ -910,6 +910,29 @@ async def test_handle_a2a_response_success_with_message(self):
       assert result.custom_metadata is not None
       assert A2A_METADATA_PREFIX + "context_id" in result.custom_metadata
 
+  @pytest.mark.asyncio
+  async def test_handle_a2a_response_message_converter_returns_none(self):
+    """Test _handle_a2a_response returns None when message converter returns None."""
+    mock_a2a_message = Mock(spec=A2AMessage)
+    mock_a2a_message.context_id = "context-123"
+
+    with patch(
+        "google.adk.agents.remote_a2a_agent.convert_a2a_message_to_event"
+    ) as mock_convert:
+      mock_convert.return_value = None
+
+      result = await self.agent._handle_a2a_response(
+          mock_a2a_message, self.mock_context
+      )
+
+      assert result is None
+      mock_convert.assert_called_once_with(
+          mock_a2a_message,
+          self.agent.name,
+          self.mock_context,
+          self.mock_a2a_part_converter,
+      )
+
   @pytest.mark.asyncio
   async def test_handle_a2a_response_with_task_completed_and_no_update(self):
     """Test successful A2A response handling with non-streaming task and no update."""
@@ -1925,6 +1948,27 @@ async def test_handle_a2a_response_impl_update_converter_returns_none(self):
         self.mock_config.a2a_part_converter,
     )
 
+  @pytest.mark.asyncio
+  async def test_handle_a2a_response_impl_message_converter_returns_none(self):
+    """Test _handle_a2a_response_v2 returns None when message converter returns None."""
+    mock_a2a_message = Mock(spec=A2AMessage)
+    mock_a2a_message.metadata = {}
+    mock_a2a_message.context_id = "context-123"
+
+    self.mock_config.a2a_message_converter.return_value = None
+
+    result = await self.agent._handle_a2a_response_v2(
+        mock_a2a_message, self.mock_context
+    )
+
+    assert result is None
+    self.mock_config.a2a_message_converter.assert_called_once_with(
+        mock_a2a_message,
+        self.agent.name,
+        self.mock_context,
+        self.mock_config.a2a_part_converter,
+    )
+
   @pytest.mark.asyncio
   async def test_handle_a2a_response_impl_unknown_response_type(self):
     """Test _handle_a2a_response_impl with unknown response type."""