Editor: Skip cross-origin isolation for third-party page builders.

Document-Isolation-Policy (DIP) isolates the document and blocks same-origin iframe access that page builders rely on. Skip DIP setup when a third-party page builder overrides the block editor via a custom `action` query parameter.

Also gates `wp_is_client_side_media_processing_enabled()` on a secure context check, since `SharedArrayBuffer` requires a secure context (HTTPS or localhost). 

Props adamsilverstein, westonruter, mukesh27, louiswol94, manhar, illuminea.
Fixes #64803.



git-svn-id: https://develop.svn.wordpress.org/trunk@61947 602fd350-edb4-49c9-b593-d223f7449a82