REST API: Restore the global post after preparing a revision

[MicahelE]

Trac: https://core.trac.wordpress.org/ticket/65495

WP_REST_Revisions_Controller::prepare_item_for_response() sets the global $post and calls setup_postdata(), but never restores them, so the change leaks for the rest of the request.

The autosaves controller delegates here, and the block editor preloads /autosaves on every load. When the post has a pending autosave, the global $post is left pointing at the autosave, so the editor can initialize with the wrong post id and redirect to it. That presents as the editor randomly opening a different post.

This captures the previous global $post and restores it on every return path (including the HEAD early return), which also covers the autosaves endpoint.

Reproduction (no browser):

wp eval '
$pid = wp_insert_post( array( "post_status" => "publish" ) );
wp_create_post_autosave( array( "post_ID" => $pid, "post_content" => "x", "post_type" => "post" ) );
global $post; $post = get_post( $pid ); setup_postdata( $post );
rest_get_server()->dispatch( new WP_REST_Request( "GET", "/wp/v2/posts/$pid/autosaves" ) );
echo $GLOBALS["post"]->ID === $pid ? "OK" : "LEAKED";
'

Before: LEAKED. After: OK.

Tests: Adds coverage for the restore (GET, HEAD, and the no-global-post case). The existing "sets up postdata" tests were asserting the leak, so they now confirm rendered fields still reflect the revision while the global post is restored.

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Props micahele.

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