REST API: Fix incorrect 403 status code for invalid post ID in Comments controller.

`update_item()` was returning HTTP 403 (Forbidden) when a non-existent
  post ID was provided. The correct status is 404 (Not Found), as the
  resource is missing — not access-restricted.

  This also makes `update_item()` consistent with `get_item()` in the
  same controller, which already returns 404 for the same scenario.

Author: saratheonline

src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php

@@ -920,7 +920,7 @@ public function update_item( $request ) {
 				return new WP_Error(
 					'rest_comment_invalid_post_id',
 					__( 'Invalid post ID.' ),
-					array( 'status' => 403 )
+					array( 'status' => 404 )
 				);
 			}
 		}