selectAlternativeVideoCodec walked the enabled codecs and returned the
first one with a video/ prefix. video/rtx and video/flexfec-03 both have
that prefix, so when the requested codec was not enabled the track could
fall back to RTX. RTX is a retransmission stream, not a real codec, so
the track got registered as video/rtx and subscribers saw a blank tile.
Skip RTX and FEC entries so the fallback is always a valid video codec.
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>