Salvaged from PR #47588 and rebased onto the post-campaign streaming core:
the StreamingTTSProvider ABC/registry and the ElevenLabs/OpenAI streamers
already live on main (tools/tts_streaming.py), so this ports the pieces
main lacked:
- GeminiStreamer: streamGenerateContent?alt=sse -> base64 PCM chunks
(24 kHz mono int16), reusing main's DEFAULT_GEMINI_TTS_* constants.
- XAIStreamer: WebSocket wss://api.x.ai/v1/tts -> binary PCM frames,
async->sync bridged via the _collect_async test seam.
- tts.streaming.provider config knob: pin one streamer, or 'auto' to
walk the priority list elevenlabs -> gemini -> openai -> xai. Unset
keeps the never-swap-the-user's-voice default.
- docs/streaming-tts.md: architecture, capability matrix, how to add
a provider.
- Unit tests for the knob, SSE parsing, and the WS bridge; key-gated
E2E tests (skipped without credentials).
Refs: #47588
