test: fix dev task relay usage audit mock setup

Author: terrywangcode

server/tests/test_gateway/test_dev_task_relay.py

@@ -16,6 +16,9 @@ def _session_factory(session: AsyncMock | None = None):
     if session is None:
         session = AsyncMock()
     session.add = MagicMock()
+    execute_result = MagicMock()
+    execute_result.scalar_one_or_none.return_value = None
+    session.execute = AsyncMock(return_value=execute_result)
 
     @asynccontextmanager
     async def factory():