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.

examples/voice/voice-xai.py

@@ -22,8 +22,8 @@ from pipecat.processors.aggregators.llm_response_universal import (
 )
 from pipecat.runner.types import RunnerArguments
 from pipecat.runner.utils import create_transport
-from pipecat.services.deepgram.stt import DeepgramSTTService
 from pipecat.services.xai.llm import GrokLLMService
+from pipecat.services.xai.stt import XAISTTService
 from pipecat.services.xai.tts import XAIHttpTTSService
 from pipecat.transports.base_transport import BaseTransport, TransportParams
 from pipecat.transports.daily.transport import DailyParams
@@ -53,7 +53,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     logger.info(f"Starting bot")
 
     async with aiohttp.ClientSession() as session:
-        stt = DeepgramSTTService(api_key=os.getenv("DEEPGRAM_API_KEY"))
+        stt = XAISTTService(api_key=os.getenv("XAI_API_KEY"))
 
         tts = XAIHttpTTSService(
             api_key=os.getenv("XAI_API_KEY"),