Update connection check to use is_alive method in StompBroker

Author: vrslev

packages/faststream-stomp/faststream_stomp/broker.py

@@ -114,7 +114,7 @@ async def ping(self, timeout: float | None = None) -> bool:
                 if cancel_scope.cancel_called:
                     return False
 
-                if self._connection._connection_manager._active_connection_state:  # noqa: SLF001
+                if self._connection.is_alive():
                     return True
 
                 await anyio.sleep(sleep_time)  # pragma: no cover