Media: Sideload and clean up the web-safe companion of a transcoded video upload

[adamsilverstein]

Trac ticket: https://core.trac.wordpress.org/ticket/65998

Backport of the PHP from WordPress/gutenberg#79375. Follow up to #12005 / [62623], which added the animated GIF video companions this builds on.

When client-side media processing is enabled, the editor transcodes an uploaded video that is not already web-safe to MP4/WebM in the browser and sideloads the result as a companion of the original video attachment, which stays the attachment. This adds the server side of that flow:

• Accept a video attachment as the sideload parent for the new optimized_video companion size, and only for that size. Every other size still needs an image or PDF parent.
• Record the companion under the optimized_video metadata key on finalize, and include it in the names a request is allowed to store.
• Delete the companion alongside the attachment in wp_delete_attachment_files().
• Expose a wp_video_transcoding_keep_original filter (default true) on the REST index as video_keep_original, and preload it with the other media processing settings.

Note: the Gutenberg PR originally used optimized-video (hyphen) for the size token. Every other special size uses underscores, so the token was renamed to optimized_video on the Gutenberg side to match this backport.

Testing

• npm run test:php -- --filter 'Tests_Media_wpDeleteAttachmentOptimizedVideo|optimized_video|test_get_index_should'
• All new tests were confirmed to fail before the source changes and pass after.

AI Use

Code and description both written with 🤖 Claude Code. I will review and test.

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