fix: stop a subscriber answer from advertising AV1 on firefox
The same gate for the dual peer connection path, where subscribed media
arrives on transceivers the server's offer creates rather than on
pre-populated sections. Only the subscriber connection answers an offer,
and only the transceivers it receives on are narrowed, so a transceiver
that sends keeps every codec it could send.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>