fix(helpdesk): use die() instead of echo for file proxy output

MintyPHP Analyzer rejects echo in action files. Use die() which is
the same pattern as Router::download() internally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/pages/helpdesk/ticket-file-data().php

@@ -77,9 +77,10 @@ header('Content-Type: ' . $mimeType);
 header('Content-Disposition: ' . $disposition . '; filename="' . addcslashes($displayFilename, '"\\') . '"');
 header('Content-Length: ' . strlen($data));
 header('Cache-Control: private, max-age=3600');
+header('Content-Transfer-Encoding: Binary');
 
 while (ob_get_level()) {
     ob_end_clean();
 }
 
-echo $data;
+die($data);