fix(proxy): drop the interrupt that Jupyter sends before a shutdown

Jupyter interrupts a kernel before it shuts it down. The proxy forwarded that interrupt_request to the shared service kernel, so stopping a server or closing a service console stopped code that ran in the service, also code that other users started.

The proxy now holds each interrupt_request for 0.5 s. If a shutdown_request arrives in that time, the proxy drops the interrupt. Otherwise it forwards the interrupt, so the Interrupt button still works.

Related to !531 (merged)

Closes #425 (closed)

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