Fix 39c serverless example: bind mock server inside __main__

[nthmost-orkes]

What

The 39c_serverless_code_execution.py example bound its mock execution server to port 9753 at module level:

mock_server = _start_mock_server(port=9753)

AgentRuntime re-imports the example file in its worker process. On re-import the worker hit that same line, tried to bind port 9753 a second time, and died with Address already in use — so the example never completed.

Fix

Move the bind inside the if __name__ == "__main__": guard. The worker's re-import evaluates the module but skips __main__, so it never re-binds. The Agent definition stays at module level (it only holds config and binds nothing), so the worker still imports everything it needs.

User impact

Running the serverless code-execution example would crash the moment the agent worker spun up (Address already in use), so it never produced a result. Now the port is bound once in the main process and the worker runs cleanly against it.

Fixes #512