feat: Allow injecting a custom Runner into agent_to_a2a

This change adds an optional `runner` parameter to the `to_a2a` function, enabling users to provide a pre-configured `Runner` instance instead of always using the default in-memory services. A new test case has been added to verify this functionality.

closes #3104

Co-authored-by: Dongyu Jia <dongyuj@google.com>
PiperOrigin-RevId: 826526861

Author: dongyuj1

src/google/adk/a2a/utils/agent_to_a2a.py

@@ -90,6 +90,7 @@ def to_a2a(
     port: int = 8000,
     protocol: str = "http",
     agent_card: Optional[Union[AgentCard, str]] = None,
+    runner: Optional[Runner] = None,
 ) -> Starlette:
   """Convert an ADK agent to a A2A Starlette application.
 
@@ -101,6 +102,8 @@ def to_a2a(
       agent_card: Optional pre-built AgentCard object or path to agent card
                   JSON. If not provided, will be built automatically from the
                   agent.
+      runner: Optional pre-built Runner object. If not provided, a default
+              runner will be created using in-memory services.
 
   Returns:
       A Starlette application that can be run with uvicorn
@@ -132,7 +135,7 @@ async def create_runner() -> Runner:
   task_store = InMemoryTaskStore()
 
   agent_executor = A2aAgentExecutor(
-      runner=create_runner,
+      runner=runner or create_runner,
   )
 
   request_handler = DefaultRequestHandler(

tests/unittests/a2a/utils/test_agent_to_a2a.py

@@ -103,6 +103,51 @@ def test_to_a2a_default_parameters(
         "startup", mock_app.add_event_handler.call_args[0][1]
     )
 
+  @patch("google.adk.a2a.utils.agent_to_a2a.A2aAgentExecutor")
+  @patch("google.adk.a2a.utils.agent_to_a2a.DefaultRequestHandler")
+  @patch("google.adk.a2a.utils.agent_to_a2a.InMemoryTaskStore")
+  @patch("google.adk.a2a.utils.agent_to_a2a.AgentCardBuilder")
+  @patch("google.adk.a2a.utils.agent_to_a2a.Starlette")
+  def test_to_a2a_with_custom_runner(
+      self,
+      mock_starlette_class,
+      mock_card_builder_class,
+      mock_task_store_class,
+      mock_request_handler_class,
+      mock_agent_executor_class,
+  ):
+    """Test to_a2a with a custom runner."""
+    # Arrange
+    mock_app = Mock(spec=Starlette)
+    mock_starlette_class.return_value = mock_app
+    mock_task_store = Mock(spec=InMemoryTaskStore)
+    mock_task_store_class.return_value = mock_task_store
+    mock_agent_executor = Mock(spec=A2aAgentExecutor)
+    mock_agent_executor_class.return_value = mock_agent_executor
+    mock_request_handler = Mock(spec=DefaultRequestHandler)
+    mock_request_handler_class.return_value = mock_request_handler
+    mock_card_builder = Mock(spec=AgentCardBuilder)
+    mock_card_builder_class.return_value = mock_card_builder
+    custom_runner = Mock(spec=Runner)
+
+    # Act
+    result = to_a2a(self.mock_agent, runner=custom_runner)
+
+    # Assert
+    assert result == mock_app
+    mock_starlette_class.assert_called_once()
+    mock_task_store_class.assert_called_once()
+    mock_agent_executor_class.assert_called_once_with(runner=custom_runner)
+    mock_request_handler_class.assert_called_once_with(
+        agent_executor=mock_agent_executor, task_store=mock_task_store
+    )
+    mock_card_builder_class.assert_called_once_with(
+        agent=self.mock_agent, rpc_url="http://localhost:8000/"
+    )
+    mock_app.add_event_handler.assert_called_once_with(
+        "startup", mock_app.add_event_handler.call_args[0][1]
+    )
+
   @patch("google.adk.a2a.utils.agent_to_a2a.A2aAgentExecutor")
   @patch("google.adk.a2a.utils.agent_to_a2a.DefaultRequestHandler")
   @patch("google.adk.a2a.utils.agent_to_a2a.InMemoryTaskStore")