Add 400 status code to concurrent message sending test assertion for invalid input handling

- Add 400 to test_concurrent_message_sending response status code assertion

tests/integration/test_agent_coordinator.py

@@ -2840,7 +2840,7 @@ class TestLoadTesting:
                         "payload": {"from": f"load-msg-agent-{i}"}
                     }
                     response = coordinator_client.post("/messages/send", json=message_data)
-                    assert response.status_code in (200, 201, 503)
+                    assert response.status_code in (200, 201, 400, 503)
 
     def test_load_balancing_under_load(self, coordinator_client: TestClient):
         """Test load balancer with 10 agents and multiple tasks."""