fix: clean up _video_drains when track ends without subscriber

If a participant leaves before anyone subscribes to their video,
the drain entry stays pinned in _video_drains for the lifetime
of the peer connection.

Author: aliev

getstream/video/rtc/pc.py

@@ -231,6 +231,7 @@ def handle_track_ended(self, track: aiortc.mediastreams.MediaStreamTrack) -> Non
             del self.track_map[track.id]
         if track.id in self.video_frame_trackers:
             del self.video_frame_trackers[track.id]
+        self._video_drains.pop(track.id, None)
 
     def get_video_frame_tracker(self) -> Optional[Any]:
         """Get a video frame tracker for stats collection.