fix: prevent "Bad state: Cannot add event after closing" in MdkVideoPlayer

[Copilot]

After dispose() closes the StreamController, async and native callbacks can still fire and attempt to add events, throwing Bad state: Cannot add event after closing.

Changes

• lib/src/video_player_mdk.dart — Added streamCtl.isClosed guards before every streamCtl.add() and streamCtl.addError() call:
  • textureSize.then(...) async callback (the primary race: future resolves after dispose)
  • onMediaStatus listener — bufferingStart / bufferingEnd events
  • onEvent listener — bufferingUpdate event
  • onStateChanged listener — completed / isPlayingStateUpdate events
  • create() error paths — media open failure and invalid video size
  • getPosition() — bufferingUpdate event

textureSize.then((size) {
  if (size == null) return;
  if (streamCtl.isClosed) return;  // added
  streamCtl.add(VideoEvent(...));
});

Events arriving after disposal are silently dropped rather than throwing.

• Fixes Unhandled Exception: Bad state: Cannot add event after closing #390