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/configuration/check_validity.py

@@ -64,6 +64,7 @@ class UserConfigurationValidator:
             ServiceProviders.RIME.value: self._check_rime_api_key,
             ServiceProviders.MINIMAX.value: self._check_minimax_api_key,
             ServiceProviders.SMALLEST.value: self._check_smallest_api_key,
+            ServiceProviders.XAI.value: self._check_xai_api_key,
         }
 
     async def validate(
@@ -376,6 +377,32 @@ class UserConfigurationValidator:
     def _check_grok_realtime_api_key(self, model: str, api_key: str) -> bool:
         return True
 
+    def _check_xai_api_key(self, model: str, api_key: str) -> bool:
+        # Use the TTS voices endpoint as a best-effort smoke test. Some xAI keys
+        # can be scoped in ways that block listing voices even though the key is
+        # still intended for TTS usage, so only a clear auth failure rejects save.
+        try:
+            response = httpx.get(
+                "https://api.x.ai/v1/tts/voices",
+                headers={"Authorization": f"Bearer {api_key}"},
+                timeout=10.0,
+            )
+        except httpx.RequestError:
+            raise ValueError(
+                "Could not connect to the xAI API. Please check your network "
+                "connection and try again."
+            )
+        if response.status_code == 200:
+            return True
+        if response.status_code == 401:
+            raise ValueError(
+                "Invalid xAI API key. The key was rejected by the xAI API. "
+                "Please check that your API key is correct and active. "
+                "You can verify your keys at "
+                "https://console.x.ai."
+            )
+        return True
+
     def _check_ultravox_realtime_api_key(self, model: str, api_key: str) -> bool:
         return True