[TOOLS] Fix errors pointed out by PHPStan level 4

components/Posting/Posting.php

@@ -33,6 +33,7 @@ use App\Core\Router;
 use App\Core\VisibilityScope;
 use App\Entity\Activity;
 use App\Entity\Actor;
+use App\Entity\LocalUser;
 use App\Entity\Note;
 use App\Util\Common;
 use App\Util\Exception\BugFoundException;
@@ -180,7 +181,9 @@ class Posting extends Component
     ): array {
         $scope ??= VisibilityScope::EVERYWHERE; // TODO: If site is private, default to LOCAL
         $reply_to_id = \is_null($reply_to) ? null : (\is_int($reply_to) ? $reply_to : $reply_to->getId());
-        $mentions    = [];
+
+        /** @var array<int, array{ mentioned?: array<int, Actor|LocalUser> }> $mentions */
+        $mentions = [];
         if (\is_null($rendered) && !empty($content)) {
             Event::handle('RenderNoteContent', [$content, $content_type, &$rendered, $actor, $locale, &$mentions]);
         }