Site editor: preload post if needed (WordPress#66631)

Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>

backport-changelog/6.8/7695.md

@@ -0,0 +1,3 @@
+https://github.com/WordPress/wordpress-develop/pull/7695
+
+* https://github.com/WordPress/gutenberg/pull/66631

lib/compat/wordpress-6.8/preload.php

@@ -10,6 +10,13 @@
  */
 function gutenberg_block_editor_preload_paths_6_8( $paths, $context ) {
 	if ( 'core/edit-site' === $context->name ) {
+		if ( ! empty( $_GET['postId'] ) ) {
+			$route_for_post = rest_get_route_for_post( $_GET['postId'] );
+			if ( $route_for_post ) {
+				$paths[] = add_query_arg( 'context', 'edit', $route_for_post );
+			}
+		}
+
 		// Core already preloads both of these for `core/edit-post`.
 		$paths[] = '/wp/v2/settings';
 		$paths[] = array( '/wp/v2/settings', 'OPTIONS' );