fix: getattr-guard _stop_loop_liveness_guards in GatewayRunner.stop

Teardown-path tests build bare runners via object.__new__ without
the liveness-guard machinery; the unguarded call raised
AttributeError in 8 tests. Same guard pattern as the start path.

gateway/run.py

@@ -9431,7 +9431,11 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
         service_restart: bool = False,
     ) -> None:
         """Stop the gateway and disconnect all adapters."""
-        self._stop_loop_liveness_guards()
+        # getattr-guard: shutdown-path tests build bare runners via
+        # object.__new__ that lack the liveness-guard machinery.
+        _stop_guards = getattr(self, "_stop_loop_liveness_guards", None)
+        if callable(_stop_guards):
+            _stop_guards()
         if restart:
             self._restart_requested = True
             self._restart_detached = detached_restart