serve_policy exposes the websocket policy server on 0.0.0.0 with no --host flag and returns tracebacks to the client

src/openpi/serving/websocket_policy_server.py defaults to host="0.0.0.0" and max_size=None, has no auth or origin check, and sends traceback.format_exc() to whoever is connected. scripts/serve_policy.py only exposes --port, so there is no way to bind to loopback without editing the source.

On a shared lab network or a workstation with a public interface, that is an unauthenticated remote call into a policy that drives an arm, plus a stack trace that leaks paths.

Suggested minimal change: add --host to scripts/serve_policy.py defaulting to 127.0.0.1, and gate the traceback behind a debug flag. That keeps every current workflow working for anyone who passes --host 0.0.0.0 deliberately.

Happy to send the PR if you want it. Checked open issues first; did not find this one.