test(loop): accept executor kwargs in subgraph async_start mock

The loop body now runs its subgraph through get_default_coordinator().stream,
so InProcessExecutor calls async_start with inputs/max_iterations/config/
reset_output_values/fallback_to_env_vars alongside event_manager. The mock
only accepted event_manager and raised TypeError on the extra kwargs. Widen it
to **_kwargs while still capturing event_manager for the assertion.

src/backend/tests/unit/components/flow_controls/test_loop.py

@@ -389,7 +389,7 @@ class TestLoopComponentSubgraphExecution:
         event_manager_received = []
 
         # Create an async generator that yields mock results
-        async def mock_async_start(event_manager=None):
+        async def mock_async_start(event_manager=None, **_kwargs):
             event_manager_received.append(event_manager)
             yield MagicMock(valid=True, result_dict={"outputs": {}})