Flush the STT buffer on a leg without stopping STT

Forces the provider to emit a final transcript for the audio buffered so far while the session keeps running, so a caller that knows the speaker has finished does not have to wait for the provider's own endpointing. Only Deepgram supports this; the Azure and ElevenLabs integrations answer 501. The flushed transcript arrives on the usual stt.text event with is_final true — a segment containing no speech produces no event at all, so do not block on one.