Add xAI TTS service

[niczy]

Summary

• Add XAIHttpTTSService backed by xAI's HTTP text-to-speech API
• Support all 20 xAI-supported languages via explicit LANGUAGE_MAP using resolve_language()
• Configurable output encoding (pcm, mp3, wav, mulaw, alaw) as a constructor parameter
• Add 07e-interruptible-xai.py foundational example; update 14g-function-calling-grok.py to use xAI TTS
• Register 07e in run-release-evals.py

Testing

• uv run pytest tests/test_xai_tts.py
• Test live with examples/foundational/07e-interruptible-xai.py using a valid GROK_API_KEY

[markbackman]

Thanks for the contribution! Here are some changes needed before we can merge:

1. Rename XAITTSService to XAIHttpTTSService — This is an HTTP-based service, and we want to leave room for a future WebSocket implementation. This follows the pattern of ElevenLabsHttpTTSService / CartesiaHttpTTSService. Update the settings class name to XAIHttpTTSSettings as well.
2. Add language map — language_to_service_language currently does return str(language) which passes through any Language enum value verbatim. xAI only supports 20 specific language codes (some regional like ar-EG, pt-BR, es-MX). Please add a language_to_xai_language() function with an explicit LANGUAGE_MAP using resolve_language(), like other services do (see cartesia/tts.py or elevenlabs/tts.py for examples). Also needs from pipecat.transcriptions.language import Language, resolve_language.
3. Remove deprecated InputParams, params, voice, language init params — Since this is a brand new service that has never shipped, there's nothing to be backward-compatible with.
4. Remove class constants XAI_DEFAULT_SAMPLE_RATE and XAI_PCM_CODEC - instead, we want to include encoding as a constructor parameter and set sample_rate to None, so that it can be set either by default from the PipelineParams or by the user via a constructor parameter.
5. Use Language.EN instead of string "en" for default language.
6. README link was missing; docs will be automatically generated after this is merged. Location will be https://docs.pipecat.ai/server/services/tts/xai
7. Linting fixes.
8. Examples: Added 07e foundational example; updated 14g foundational example.

I made a branch while I was reviewing these changes. I've pushed these to origin for you to use as a starting point:
#4121

Also, once this change is in, I think we'll move the GrokLLMService to the new xai module, so that services are consolidate. We can do that in a separate PR though.