feat(tts): add websocket transport option for xAI TTS (#548)

* feat(tts): add websocket transport option for xAI TTS

xAI's pipecat service ships two implementations: XAIHttpTTSService
(batch REST, used by the existing xAI integration) and XAITTSService
(realtime WebSocket streaming). This adds a transport field on
XAITTSConfiguration ("http", default, unchanged behavior | "websocket")
and wires the latter into create_tts_service(), so live voice calls can
opt into lower time-to-first-byte streaming instead of request/response.

Originally built and battle-tested independently against v1.41.0 in a
production deployment (7 live Telnyx calls, 2026-07-15) before this PR
existed; ported onto current main and given a config toggle instead of
a second provider entry so it composes with the existing xAI HTTP path
rather than duplicating it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* refactor(tts): use websocket-only xAI TTS, drop the transport toggle

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Sabiha Khan <sabihak89@gmail.com>

api/services/pipecat/service_factory.py

@@ -86,7 +86,7 @@ from pipecat.services.speechmatics.stt import (
     SpeechmaticsSTTService,
     SpeechmaticsSTTSettings,
 )
-from pipecat.services.xai.tts import XAIHttpTTSService, XAITTSSettings
+from pipecat.services.xai.tts import XAITTSService, XAIWebsocketTTSSettings
 from pipecat.transcriptions.language import Language
 from pipecat.utils.text.xml_function_tag_filter import XMLFunctionTagFilter
 
@@ -817,11 +817,9 @@ def create_tts_service(
                 pipecat_language = Language(language_code)
             except ValueError:
                 pipecat_language = Language.EN
-        return XAIHttpTTSService(
+        return XAITTSService(
             api_key=user_config.tts.api_key,
-            sample_rate=audio_config.transport_out_sample_rate,
-            encoding="pcm",
-            settings=XAITTSSettings(
+            settings=XAIWebsocketTTSSettings(
                 voice=voice,
                 language=pipecat_language,
             ),