Reworks the xAI TTS integration from #4031 with consistency fixes:
- Rename to XAIHttpTTSService (leaves room for future WebSocket service)
- Add proper language map with all 20 supported xAI languages
- Remove unnecessary deprecated InputParams/params (new service, nothing to deprecate)
- Add encoding as a constructor parameter
- Use Language.EN enum instead of string for default language
- Linting fixes