chore: Upgrade the sample BQ agent model version to gemini-2.5-flash

This change should give a more intelligent out-of-the-box experience to the sample BigQuery agent users.

PiperOrigin-RevId: 855552871

Author: google-genai-bot

contributing/samples/bigquery/agent.py

@@ -71,7 +71,7 @@
 # The variable name `root_agent` determines what your root agent is for the
 # debug CLI
 root_agent = LlmAgent(
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     name=BIGQUERY_AGENT_NAME,
     description=(
         "Agent to answer questions about BigQuery data and models and execute"