fix: handle userid=0 for team submissions in process_feedback_adhoc

  core_user::get_user(0) returns false for group submissions; passing
  false to setup_user(?stdClass) throws a TypeError in PHP 8. Convert
  false to null and move setup_user inside try/finally for safe cleanup.

Author: marcusgreen

classes/task/process_feedback_adhoc.php

@@ -120,10 +120,13 @@ private function generate_feedback($record, string $triggeredby = 'manual', ?\as
 
         // Set up the user context for the submission owner so that quota and
         // availability checks are performed against the correct user.
-        $submissionuser = \core_user::get_user($record->userid);
-        \core\cron::setup_user($submissionuser);
+        // For team submissions, userid may be 0 (group submission); get_user()
+        // returns false in that case, so fall back to null (cron default user).
+        $submissionuser = \core_user::get_user($record->userid) ?: null;
 
         try {
+            \core\cron::setup_user($submissionuser);
+
             if (!$provider->is_available($purpose, $record->contextid)) {
                 mtrace("AI backend not available for user {$record->userid}, skipping submission {$record->subid}.");
                 return;