fix(xai/realtime): pass model as query param on connect

xAI's Voice Agent API selects the model via the ?model= query
parameter on the WebSocket URL; it cannot be changed later via
session.update. The Grok Realtime service was setting the model in
Settings but never including it in the connection URL, so every
session silently fell back to the deprecated default
grok-voice-fast-1.0.

Append the model from Settings to the WebSocket URL on connect, and
default to the recommended grok-voice-think-fast-1.0.

src/pipecat/services/xai/realtime/llm.py

@@ -17,6 +17,7 @@ from collections.abc import Mapping
 from dataclasses import dataclass, field
 from dataclasses import fields as dataclass_fields
 from typing import Any
+from urllib.parse import quote
 
 from loguru import logger
 
@@ -235,7 +236,7 @@ class GrokRealtimeLLMService(LLMService):
         """
         # 1. Initialize default_settings with hardcoded defaults
         default_settings = self.Settings(
-            model=None,
+            model="grok-voice-think-fast-1.0",
             system_instruction=None,
             temperature=None,
             max_tokens=None,
@@ -533,8 +534,13 @@ class GrokRealtimeLLMService(LLMService):
             if self._websocket:
                 return
 
+            # Model is selected via query param at connection time; xAI does
+            # not support changing it via session.update.
+            model = assert_given(self._settings.model)
+            uri = f"{self.base_url}?model={quote(model, safe='')}"
+
             self._websocket = await websocket_connect(
-                uri=self.base_url,
+                uri=uri,
                 additional_headers={
                     "Authorization": f"Bearer {self.api_key}",
                 },