fix: Fix import for live bidi streaming single agent

While testing the bidi streaming sample agent, I noticed that the import was erroring and I think it's a typo -- the other bidi sample agents all import `from google.adk.agents.llm_agent`.

PiperOrigin-RevId: 823662586

Author: wukath

contributing/samples/live_bidi_streaming_single_agent/agent.py

@@ -14,7 +14,7 @@
 
 import random
 
-from google.adk.agents.agent import Agent
+from google.adk.agents.llm_agent import Agent
 from google.adk.tools.tool_context import ToolContext
 from google.genai import types