Join link REST resource returns 400 with internal messages instead of 403/422

Problem/Motivation

In BigBlueButtonJoinMeetingLink::get() the catch (\RuntimeException) comes first. UnprocessableEntityHttpException extends \RuntimeException, so the 422 branch can never run. "No access" and "not found" both come back as 400,
with the internal exception message in the response.

Proposed resolution

Catch HttpExceptionInterface first and keep its status code behind a generic message (the same pattern BigBlueButtonGetMeetingInfo uses). Log anything else and answer 500.