Ensure REST API endpoint for side-loaded media covers all requirements #74360
Description
What problem does this address?
The media experiment in Gutenberg already adds a new REST API endpoint (wp/v2/media//sideload) that accepts pre-processed images with their sub-sizes, bypassing server-side processing. The purpose of this ticket is to ensure the endpoints supports all current requirements.
Acceptance Criteria:
- New POST endpoint at
wp/v2/media/<id>/sideload - Validates uploaded sizes match expected dimensions
- Updates attachment metadata appropriately, populating image meta
- Proper permission checks