fix(proxy): stop forwarding shutdown_request to the shared service kernel

Jupyter sends a shutdown_request to a kernel whenever it is shut down, including when a notebook is closed or the user stops their server. The proxy forwarded it to the real service kernel, which then exited and was respawned by supervisor, restarting it for every user.

Intercept shutdown_request in the proxy, reply on the service's behalf and exit only the proxy process. Genuine restarts are still detected on reconnect and restart the service once.

Closes #419 (closed)

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com