[video_player] Add background playback with media notification support

[Deni-al]

Adds system media notification support for background playback, allowing users to control video playback from the lock screen, notification shade, and control center.

Features:

• New NotificationMetadata class for customizing media notification content
• New notificationMetadata parameter in VideoPlayerOptions to enable notifications
• updateNotificationMetadata() method for dynamic metadata updates during playback
• System media controls (play/pause/seek) on lock screen and control center

Platform support:

• Android: Uses Media3 MediaSession with foreground service
• iOS/macOS: Uses MPNowPlayingInfoCenter and MPRemoteCommandCenter
• Web: Not supported (returns false for availability check)

Fixes flutter/flutter#62739

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request introduces background playback with system media notification support for the video_player plugin across Android, iOS, and macOS. The web platform explicitly opts out of this feature. The changes include adding new API methods in video_player_platform_interface for setting and updating notification metadata, as well as platform-specific implementations for Android (using Media3 MediaSession) and iOS/macOS (using MPNowPlayingInfoCenter and MPRemoteCommandCenter). The necessary Pigeon-generated code and test cases have been updated accordingly. Resource cleanup for background playback is also handled during player disposal.

[gemini-code-assist]

The variables options and metadata are redundant here. You can directly access videoPlayerOptions and videoPlayerOptions?.notificationMetadata within the if condition, making the code slightly more concise.

    if (videoPlayerOptions != null && videoPlayerOptions.allowBackgroundPlayback) {
      await _videoPlayerPlatform.setBackgroundPlayback(
        _playerId,
        enableBackground: true,
        notificationMetadata: videoPlayerOptions.notificationMetadata,
      );
    }

[stuartmorgan-g]

Thanks for the contribution! This appears to be substantially expanding the scope of video_player (as compared to the existing PR), so the first step here would be a design document that explains the change at a high level, and obtains buy-in from the relevant code owners on having this much background playback UI support in the core plugin.

Setting to Draft status for now, since the design document review should happen before code review.