fix: Set the default response modality to AUDIO only

Currently Live API only allow one response modality although the config is defined as a list.

Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
PiperOrigin-RevId: 853107710

Author: seanzhougoogle

src/google/adk/cli/adk_web_server.py

@@ -1624,7 +1624,7 @@ async def run_agent_live(
         user_id: str,
         session_id: str,
         modalities: List[Literal["TEXT", "AUDIO"]] = Query(
-            default=["TEXT", "AUDIO"]
+            default=["AUDIO"]
         ),  # Only allows "TEXT" or "AUDIO"
     ) -> None:
       await websocket.accept()