Client freezes instead of crashing when triggering a debug crash (F3+C) while the integrated server is frozen

If you force a crash using F3+C while the server is frozen, the client freezes instead of crashing and must be force quit. This only seems to happen in singleplayer, probably because F3+C attempts to close the integrated server as well as the client. Most likely the solution involves moving server shutdown logic to its own thread.

To reproduce

1. Freeze the server somehow (using eg. MC-269520)
2. Hold down F3+C for 10 seconds
3. Observe how the client window remains on screen, frozen
4. Navigate to the launcher. If the log is open, notice how it registered an F3+C crash as expected.
5. Force-quit the client

If the integrated server is frozen only temporarily (using eg. /locate biome mushroom_fields), the client will crash after the command completes.

Discovered while testing MC-269520.