chore: Update default model in live bidi streaming sample

Switched the active model from `gemini-live-2.5-flash-preview` (for AI Studio) to `gemini-2.0-flash-live-preview-04-09` (for Vertex).

PiperOrigin-RevId: 806348640

Author: hangfei

contributing/samples/live_bidi_streaming_multi_agent/agent.py

@@ -100,8 +100,8 @@ def get_current_weather(location: str):
 
 root_agent = Agent(
     # find supported models here: https://google.github.io/adk-docs/get-started/streaming/quickstart-streaming/
-    # model="gemini-2.0-flash-live-preview-04-09",  # for Vertex project
-    model="gemini-live-2.5-flash-preview",  # for AI studio key
+    model="gemini-2.0-flash-live-preview-04-09",  # for Vertex project
+    # model="gemini-live-2.5-flash-preview",  # for AI studio key
     name="root_agent",
     instruction="""
       You are a helpful assistant that can check time, roll dice and check if numbers are prime.