jabber notification-template prefix "short" breaks resolution of paths in extensions

When jabber is enabled in a board, it can also be used for notifications created by extensions. For all Notifications that provide an email template, a user can also choose jabber for delivery. However, the creation of the message fails with:

I have tested with different extensions as well, the error stays the same, regardless of whether a file language/de/email/short/mail_ext_update.txt is present or not (obviously, substitute mail_ext_update.txt by any other file that might be used by an extension).

In phpbb/notification/method/jabber.php, a prefix is defined for the templates, which is normally used to have shorter texts for the messages that are sent out:

return $this->notify_using_messenger(NOTIFY_IM, 'short/');

Apparently, the extension information must be at the start of the template path and therefore Twig cannot find the jabber template.

I have tested this on the console by running a cronjob that creates a notification, but creating a different notification from a different extension using a regular browser results in the same problem. Sending out emails with the same notification works without issues.

I see multiple problems here:
1. There may not be a special jabber template for each case, the notification provides an email template, but there is no fallback on a regular email template.
2. Apparently, the prefix "short/" completely breaks path resolution to find the email templates in the extensions.

Steps to reproduce:

• Install an extension that creates a notification and provides an email template for it
• activate jabber on your test board
• Enable Jabber for the notification created by that extension
• trigger the creation of the notification