fix: restore bigquery sample agent to runnable form

A previous change of import paths had rendered the agent not-runnable out of the box. This change fixes that. PiperOrigin-RevId: 788221276

Author: google-genai-bot

contributing/samples/bigquery/agent.py

@@ -62,7 +62,7 @@
 
 # The variable name `root_agent` determines what your root agent is for the
 # debug CLI
-root_agent = llm_agent.Agent(
+root_agent = LlmAgent(
  model="gemini-2.0-flash",
  name="bigquery_agent",
  description=(