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.

uv.lock

@@ -4403,6 +4403,9 @@ websockets-base = [
 whisper = [
     { name = "faster-whisper" },
 ]
+xai = [
+    { name = "websockets" },
+]
 
 [package.dev-dependencies]
 dev = [
@@ -4507,6 +4510,7 @@ requires-dist = [
     { name = "pipecat-ai", extras = ["websockets-base"], marker = "extra == 'soniox'" },
     { name = "pipecat-ai", extras = ["websockets-base"], marker = "extra == 'ultravox'" },
     { name = "pipecat-ai", extras = ["websockets-base"], marker = "extra == 'websocket'" },
+    { name = "pipecat-ai", extras = ["websockets-base"], marker = "extra == 'xai'" },
     { name = "pipecat-ai-small-webrtc-prebuilt", marker = "extra == 'runner'", specifier = ">=2.4.0" },
     { name = "piper-tts", marker = "extra == 'piper'", specifier = ">=1.3.0,<2" },
     { name = "protobuf", specifier = ">=6.31.1,<7" },