feat(tts): add xAI as a Voice (TTS) provider (#476)

* feat(tts): add xAI as a Voice (TTS) provider

pipecat already ships an xAI TTS service (XAITTSService, WebSocket
streaming) but dograh never wired it into the service configuration, so
xAI could not be selected as a Voice provider in the cascading pipeline.

Wire it through:
- registry: ServiceProviders.XAI + XAITTSConfiguration (voices
  eve/ara/leo/rex/sal, language, computed model) registered in TTSConfig
- service_factory: build XAITTSService in create_tts_service
- check_validity: api-key validation hook
- tests for the factory + docs

The Voice provider dropdown is schema-driven, so xAI appears with no UI
changes.

* fix(tts): validate xAI API key and drop misleading auto-language hint

Addresses review feedback on the xAI Voice provider:

- check_validity: replace the no-op xAI key check with real validation
  against xAI's OpenAI-compatible API (models.list on https://api.x.ai/v1),
  so a bad BYOK key is caught at configuration time instead of at call time.
- registry: remove the "auto" language hint from the field description.
  pipecat's Language enum has no "auto" member, so the factory fell back to
  English silently; the description no longer advertises detection we don't do.
- tests: cover xAI key validation (registered, accepts valid, rejects bad).

* fix(tts): validate xAI key against the TTS voices endpoint

xAI supports endpoint-scoped API keys, so a key scoped to Text-to-Speech
may lack the /v1/models ACL and would be wrongly rejected by the previous
models.list() check. Validate against GET /v1/tts/voices instead — the
scope the key actually needs for TTS — treating 401/403 as an invalid key
and connection errors as a clean, actionable message.

* fix: harden xAI TTS integration

---------

Co-authored-by: Sabiha Khan <sabihak89@gmail.com>
Co-authored-by: Sabiha Khan <87858386+chewwbaka@users.noreply.github.com>

api/services/pipecat/service_factory.py

@@ -81,6 +81,7 @@ from pipecat.services.speechmatics.stt import (
     SpeechmaticsSTTService,
     SpeechmaticsSTTSettings,
 )
+from pipecat.services.xai.tts import XAIHttpTTSService, XAITTSSettings
 from pipecat.transcriptions.language import Language
 from pipecat.utils.text.xml_function_tag_filter import XMLFunctionTagFilter
 
@@ -740,6 +741,28 @@ def create_tts_service(
             skip_aggregator_types=["recording_router", "recording"],
             silence_time_s=1.0,
         )
+    elif user_config.tts.provider == ServiceProviders.XAI.value:
+        voice = getattr(user_config.tts, "voice", None) or "eve"
+        language_code = getattr(user_config.tts, "language", None) or "en"
+        if language_code.lower() == "auto":
+            pipecat_language = "auto"
+        else:
+            try:
+                pipecat_language = Language(language_code)
+            except ValueError:
+                pipecat_language = Language.EN
+        return XAIHttpTTSService(
+            api_key=user_config.tts.api_key,
+            sample_rate=audio_config.transport_out_sample_rate,
+            encoding="pcm",
+            settings=XAITTSSettings(
+                voice=voice,
+                language=pipecat_language,
+            ),
+            text_filters=[xml_function_tag_filter],
+            skip_aggregator_types=["recording_router", "recording"],
+            silence_time_s=1.0,
+        )
     else:
         raise HTTPException(
             status_code=400, detail=f"Invalid TTS provider {user_config.tts.provider}"