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>

ui/src/client/types.gen.ts

@@ -561,7 +561,9 @@ export type ByokPipelineAiModelConfiguration = {
         provider: 'azure_speech';
     } & AzureSpeechTtsConfiguration) | ({
         provider: 'smallest';
-    } & SmallestAittsConfiguration);
+    } & SmallestAittsConfiguration) | ({
+        provider: 'xai';
+    } & XaittsConfiguration);
     /**
      * Stt
      */
@@ -7156,6 +7158,32 @@ export type WorkflowVersionResponse = {
     } | null;
 };
 
+/**
+ * xAI
+ */
+export type XaittsConfiguration = {
+    /**
+     * Provider
+     */
+    provider?: 'xai';
+    /**
+     * Api Key
+     */
+    api_key: string | Array<string>;
+    /**
+     * Voice
+     *
+     * xAI voice persona.
+     */
+    voice?: string;
+    /**
+     * Language
+     *
+     * BCP-47 language code for synthesis (e.g. 'en', 'fr', 'de'), or 'auto' for automatic language detection.
+     */
+    language?: string;
+};
+
 export type InitiateCallApiV1TelephonyInitiateCallPostData = {
     body: InitiateCallRequest;
     headers?: {