Home Assistant sends MEDIA_PLAYER_COMMAND_STOP when the queue is cleared.
The MediaPlayer entity did not handle this command, causing playback to
continue without media metadata and leaving the UI in an inconsistent state.
This change adds explicit handling for the STOP command:
- stop playback via the music player
- reset the media player state to IDLE
This aligns the behavior with the original player and Home Assistant’s
expected media player semantics.