The default output format pcm_44100 requires an ElevenLabs Pro tier
subscription. Users on free or starter plans get a silent 403 failure
and hear no audio.
Instead of hardcoding mp3, keep pcm_44100 as the default (better
quality for Pro users) but remember the failure: when a PCM request
is rejected, set pcmFormatUnavailable and use mp3_44100_128 for all
subsequent requests in the session. The flag resets on config reload
so it re-probes after reconnection.
Also standardize the MP3 fallback format from mp3_44100 to
mp3_44100_128 for consistent bitrate.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>