Fix term_exists() check and remove unused variable

- Use `! is_array()` instead of `=== false` for `term_exists()` since it
  never returns a bool (returns null|int|array|0).
- Remove unused `$taxonomy` variable in taxonomy permission check.
- Update MAX_UPDATE_DATA_SIZE docblock: enforced via maxLength (character
  count), not byte size.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chriszarate

src/wp-includes/collaboration/class-wp-http-polling-sync-server.php

@@ -54,7 +54,7 @@ class WP_HTTP_Polling_Sync_Server {
 	const MAX_ROOMS_PER_REQUEST = 50;
 
 	/**
-	 * Maximum size (in bytes) of a single update data string.
+	 * Maximum length of a single update data string.
 	 *
 	 * @since 7.0.0
 	 * @var int
@@ -357,11 +357,10 @@ private function can_user_sync_entity_type( string $entity_kind, string $entity_
 
 			// Handle single taxonomy term entities with a defined object ID.
 			if ( 'taxonomy' === $entity_kind ) {
-				if ( term_exists( $object_id, $entity_name ) === false ) {
+				if ( ! is_array( term_exists( $object_id, $entity_name ) ) ) {
 					// Either term doesn't exist OR term is not in specified taxonomy.
 					return false;
 				}
-				$taxonomy = get_taxonomy( $entity_name );
 				return current_user_can( 'edit_term', $object_id );
 			}