Add xAI streaming STT service

New `XAISTTService` wraps xAI's real-time speech-to-text WebSocket
(`wss://api.x.ai/v1/stt`). It extends `WebsocketSTTService`, authenticates
with the `XAI_API_KEY` as a Bearer token on the WS handshake, and streams
raw audio (PCM/mu-law/A-law) with configurable interim results, endpointing,
language, multichannel, and diarization settings.

- `src/pipecat/services/xai/stt.py`: new service, settings dataclass, and
  `language_to_xai_stt_language` helper.
- `src/pipecat/services/stt_latency.py`: `XAI_TTFS_P99` default.
- `pyproject.toml` / `uv.lock`: `xai` extra now pulls in `websockets-base`.
- `README.md`: link to xAI STT in the services table.
- `examples/voice/voice-xai.py`: swap DeepgramSTTService for XAISTTService so
  the xAI voice example is fully xAI.
- `examples/transcription/transcription-xai.py`: new transcription-only
  example using the new service.

pyproject.toml

@@ -123,7 +123,7 @@ webrtc = [ "aiortc>=1.14.0,<2", "opencv-python>=4.11.0.86,<5" ]
 websocket = [ "pipecat-ai[websockets-base]", "fastapi>=0.115.6,<1" ]
 websockets-base = [ "websockets>=13.1,<16.0" ]
 whisper = [ "faster-whisper~=1.2.1" ]
-xai = []
+xai = [ "pipecat-ai[websockets-base]" ]
 
 [dependency-groups]
 dev = [