fix(gateway): Matrix and Mattermost never report as connected

Neither adapter called _mark_connected() after successful connect(),
so _running stayed False, runtime status never showed 'connected',
and /status reported them as offline even while actively processing
messages.

Add _mark_connected() calls matching the pattern used by Telegram
and DingTalk adapters.

gateway/platforms/mattermost.py

@@ -222,6 +222,7 @@ class MattermostAdapter(BasePlatformAdapter):
 
         # Start WebSocket in background.
         self._ws_task = asyncio.create_task(self._ws_loop())
+        self._mark_connected()
         return True
 
     async def disconnect(self) -> None: