feat(elevenlabs): stream eleven_v3/eleven_v3_conversational via text-to-dialogue

[ameyakhare]

Summary

Adds streaming support for eleven_v3 / eleven_v3_conversational by routing them through ElevenLabs' text-to-dialogue API — the regular text-to-speech streaming websocket (multi-stream-input) rejects these models outright today, so SynthesizeStream currently fails for them.

• ChunkedStream now posts to /v1/text-to-dialogue/stream for these models.
• New _DialogueConnection mirrors the existing _Connection (same shared-connection, multi-context design for barge-in) but speaks text-to-dialogue's multi-stream-input protocol instead.
• Waits for the server's is_final_audio_for_turn before sending close_context — sending it immediately, like the regular protocol does, races the server's teardown and silently drops the last turn's audio.
• Added eleven_v3_conversational to TTSModels (eleven_v3 was already listed but non-functional for streaming).

Quickstart guide here
Websocket documentation here

Testing

• Lint/format/mypy all pass
• 16 new unit tests (26/26 total). Verified against the live API for both models — ChunkedStream and SynthesizeStream, including concurrent streams to simulate barge-in.

[CLAassistant]

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