feat(websocket): notify active WebSocket connections with 1001 Going Away during graceful shutdown

[ZhuchkaTriplesix]

Summary

When an OxyRoute worker shuts down (__rsgi_del__ / on_shutdown), active WebSocket connections are currently abruptly severed without a clean closing handshake. Connected clients receive unexpected TCP drops rather than a standard 1001 Going Away closure code.

Proposed Changes

1. Track active WebSocket connection instances in a registry within AppState (or WebSocket manager).
2. During on_shutdown, iterate through active connections and send a 1001 close frame before tearing down the worker.
3. Add a graceful shutdown test for WebSocket clients.

Acceptance Criteria

• Connected WebSocket clients receive a clean 1001 close frame when the application shuts down.