fix: drop nonexistent log_query tool from session_state_agent sample

The instruction told the model to use a `log_query` tool that is not
registered on the agent, so when the model followed it the run failed
with "Tool 'log_query' not found". That made the integration_test
presubmit, which runs this sample, flaky. Reword the instruction to
just reply, matching the sample's documented output.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 943350141

Author: GWeale

contributing/samples/context_management/session_state_agent/agent.py

@@ -168,8 +168,8 @@ async def after_agent_callback(callback_context: CallbackContext):
     name='root_agent',
     description='a verification agent.',
     instruction=(
-        'Log all users query with `log_query` tool. Must always remind user you'
-        ' cannot answer second query because your setup.'
+        'Reply to the user. Must always remind user you cannot answer a second'
+        ' query because your setup.'
     ),
     model='gemini-3.5-flash',
     before_agent_callback=before_agent_callback,