feat: show the controls when play/pause comes from outside the controls

[Ortes]

What

When playback is toggled from outside the controls — hardware media keys (▶⏸/F8, handled natively by the browser on web), Android headset/notification controls, the MediaSession API, or the embedding app driving the VideoPlayerController — the video starts/stops but MaterialDesktopControls gives no visual feedback. Pressing space (which goes through the controls' own _playPause) reveals the controls; the media key does not.

With this change any isPlaying flip reveals the controls and restarts the hide timer, so the state change is always visible and the controls auto-hide again after hideControlsTimer.

How

_updateState (already registered as a listener on the VideoPlayerController) compares the incoming controller.value.isPlaying against the previous snapshot _latestValue — which now gets an initializer so its first read can't throw — and calls the existing _cancelAndRestartTimer() on a flip. No new state, no platform-specific code: every platform implementation funnels external play/pause into value.isPlaying (isPlayingStateUpdate), so this covers web media keys, ExoPlayer's onIsPlayingChanged, iOS Control Center, etc.

Tests

test/external_playpause_test.dart drives an externally-mutated controller and covers: reveal + auto-hide on external play, reveal + auto-hide on external pause, and no reveal on position-only updates.

dart format, dart analyze lib test, and the full flutter test suite are clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.04%. Comparing base (69a8fc7) to head (7f3bdce).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
+ Coverage   43.82%   46.04%   +2.22%     
==========================================
  Files          21       21              
  Lines        1602     1605       +3     
==========================================
+ Hits          702      739      +37     
+ Misses        900      866      -34     

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