[video_player_avfoundation] Route video over AirPlay

[a1rwulf]

usesExternalPlaybackWhileExternalScreenIsActive defaults to NO, so an AVPlayer keeps the picture on the device whenever an external screen is active. Selecting an AirPlay route therefore moved only the audio, while the receiver showed a mirrored copy of the screen rather than playing the stream itself.

allowsExternalPlayback already defaults to YES, so the two together were incoherent: external playback was permitted but never used. AVKit's own AVPlayerViewController routes to AirPlay out of the box; this brings the plugin in line with it.

Verified on an Apple TV from iOS 26.6.

Fixes: flutter/flutter#191227

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Code Review

This pull request configures the AVPlayer on iOS to route video over AirPlay when an external screen is active by setting allowsExternalPlayback and usesExternalPlaybackWhileExternalScreenIsActive to true. It also adds a unit test to verify these properties and bumps the package version to 2.12.0. There are no review comments, and I have no feedback to provide.