Media: Add an opt-in filter to generate animated image sub-sizes

[adamsilverstein]

Core backport of the server-side changes in WordPress/gutenberg#80385 (design discussion in WordPress/gutenberg#80383).

Resized versions of animated GIFs have always lost their animation - the subject of the 12-year-old Trac #28474, which stalled server-side because GD cannot preserve frames and Imagick is only available on a subset of hosts. The client-side media processing path can decode and re-encode every frame in the uploading user's browser, independent of host configuration, making animation-preserving sub-sizes feasible as a developer opt-in (they remain too expensive to be the default).

This adds the new wp_generate_animated_image_subsizes filter (boolean, default false) and exposes its value on the REST API index as animated_image_subsizes, inside the existing client-side media processing block in WP_REST_Server::get_index() (alongside image_size_threshold, only for users who can upload_files). When a site opts in, the client pipeline keeps the animation in uncropped sub-sizes of animated GIFs; cropped sizes such as thumbnail always flatten to the first frame.

The field is also added to the block editor root-index preload field lists in wp-admin/edit-form-blocks.php and wp-admin/site-editor.php - these must match packages/core-data/src/entities.js exactly for preloading to work, and the matching entities.js change is part of the Gutenberg PR. (Note: #12508 may want to do the same for image_strip_meta / image_max_bit_depth.)

The client-side consumption (editor settings plumbing, @wordpress/upload-media, and the @wordpress/vips worker restoring the all-frames load path with tuned GIF encoder settings) ships via the Gutenberg package updates in WordPress/gutenberg#80385.

Trac ticket: https://core.trac.wordpress.org/ticket/65656

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Proposed commit message

Media: Add an opt-in filter to generate animated image sub-sizes.

Sub-sizes of animated images have always been static first-frame images: GD and Imagick flatten animated images when resizing, and re-encoding every frame per sub-size is very resource intensive. The client-side media processing path can preserve animation in the uploading user's browser, independent of host Imagick availability.

Add a new wp_generate_animated_image_subsizes filter (boolean, default false) and expose it on the REST API index alongside the other client-side media processing settings, so the client pipeline can honor it for uncropped sub-sizes. The client-side consumption ships via the Gutenberg package updates in https://github.com/WordPress/gutenberg/pull/80385.

Fixes #65656. See #28474, #64804.

[github-actions]

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[mukeshpanchal27]

Suggested change

	$user_id = self::factory()->user->create( array( 'role' => 'administrator' ) );
	$this->assertIsInt( $user_id );
	wp_set_current_user( $user_id );
	wp_set_current_user( self::$admin_id );

Reuse admin that already created in wpSetUpBeforeClass