Fix context_id precedence to use 'is not None' and add session-without-service-id test

Address review feedback:
- Use explicit 'is not None' check instead of 'or' for context_id
  precedence so empty strings don't incorrectly fall through
- Add test for AgentSession() with no service_session_id, verifying
  fallback to message additional_properties context_id

Fixes #5353

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

python/packages/a2a/agent_framework_a2a/_agent.py

@@ -683,7 +683,7 @@ def _prepare_message_for_a2a(self, message: Message, *, context_id: str | None =
             role=A2ARole("user"),
             parts=parts,
             message_id=message.message_id or uuid.uuid4().hex,
-            context_id=context_id or message.additional_properties.get("context_id"),
+            context_id=context_id if context_id is not None else message.additional_properties.get("context_id"),
             metadata=metadata,
         )
 

python/packages/a2a/tests/test_a2a_agent.py

@@ -952,6 +952,24 @@ async def test_run_message_context_id_used_when_no_session(mock_a2a_client: Mock
     assert mock_a2a_client.last_message.context_id == "fallback-ctx"
 
 
+@mark.asyncio
+async def test_run_message_context_id_used_when_session_has_no_service_id(mock_a2a_client: MockA2AClient) -> None:
+    """Test fallback to additional_properties context_id when session exists but service_session_id is None."""
+    agent = A2AAgent(name="Test Agent", id="test-agent", client=mock_a2a_client, http_client=None)
+    mock_a2a_client.add_message_response("msg-ctx4", "reply")
+
+    session = AgentSession()
+    message = Message(
+        role="user",
+        contents=[Content.from_text(text="Hello")],
+        additional_properties={"context_id": "fallback-ctx"},
+    )
+    await agent.run(messages=[message], session=session)
+
+    assert mock_a2a_client.last_message is not None
+    assert mock_a2a_client.last_message.context_id == "fallback-ctx"
+
+
 # endregion