[video_player] Add Picture-in-Picture support for iOS

[akaboshinit]

Description

Adds Picture-in-Picture (PiP) support for iOS to the video_player plugin across three packages.

Fixes flutter/flutter#60048

video_player_platform_interface (6.6.0 → 6.7.0)

• Add enablePictureInPicture(), disablePictureInPicture(), startPictureInPicture(), stopPictureInPicture(), isPictureInPictureSupported(), and isPictureInPictureActive() methods
• Add pipStarted, pipStopped, pipRestoreUserInterface event types to VideoEventType

video_player_avfoundation (2.9.3 → 2.10.0)

• Integrate AVPictureInPictureController with AVPlayerLayer
• Support both texture-based and platform view players
• Implement PiP delegate callbacks (start/stop/restore UI)
• Add float-up animation workaround for texture-based players
• macOS returns stub implementations (PiP not supported on macOS)
• Requires iOS 14.2+

video_player (2.11.0 → 2.12.0)

• Add isPictureInPictureActive field to VideoPlayerValue
• Add PiP methods to VideoPlayerController
• Handle pipStarted, pipStopped, pipRestoreUserInterface events

Example app

• Add PiP demo controls to avfoundation example

Test plan

• Unit tests for all PiP platform interface methods (platform_interface)
• Unit tests for all PiP Pigeon API calls (avfoundation)
• Unit tests for PiP event conversion (avfoundation)
• Unit tests for PiP methods on VideoPlayerController (video_player)
• Unit tests for VideoPlayerValue.isPictureInPictureActive state updates (video_player)
• Unit tests for PiP methods before initialization throw StateError (video_player)
• Manual test: PiP enable/disable on iOS device
• Manual test: PiP start/stop on iOS device
• Manual test: PiP events fire correctly

Note: PiP requires AVPictureInPictureController which is only available on real iOS devices (iOS 14.2+). Native XCTest unit tests are not included as PiP functionality cannot be tested in the simulator.

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. [video_player]
• 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.

Footnotes

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