Fix thread leak hanging shutdown on 26.2

[TauCu]

Since 26.2, the client (and possibly server?) waits for threads to exit before closing. Crashing after a short time.
This pull fixes a bug where SessionManager didn't properly shut down.

The game crashed: client shutdown from post-main
Error: java.lang.Error: Watchdog (Client shutdown from post-main)

Cause:

1. SessionManager is initialized at client start but only shutdown at server shutdown. If the client never opens a world, SessionManager#unload is never called.
2. SessionManager#unload does not shutdown the executor, causing the client to time out and crash when waiting for it to exit anyway.

Fixes:

• Move SessionManager to PlatformManager and make it use platform ready/unready events.
• Fix SessionManager#unload canceling timer and no longer auto-saving after quitting a world.
• Remove timer and use a single ScheduledExecutorService with a single virtual thread.
• Make sure executor shuts down on SessionManager#unload

Draft reason:
I'm making this as a draft for now as I'm not entirely sure what consequences of making the session manager dependent on Ready/Unready event are.
I smoketested it on all platforms but that doesn't really say much.
Could someone with some knowledge of when SessionManager is supposed to be available chime in?

The only other thing I want some insight on was the effects of ScheduledExecutorService using an unbounded DelayedWorkQueue and the original used a bounded-queue of 5. Unless I'm missing something I don't think this will cause issues?

[me4502]

I'm a bit confused by this PR, is this specifically an issue on a certain platform? I've been using Fabric with WorldEdit installed while joining a server to test the Bukkit version of WorldEdit on 26.2 and never hit this issue, and from a quick test of just launching the game in both NF & Fabric to the menu and closing it I cannot reproduce any issues.

Can you please elaborate on what about 26.2 has caused this problem?

[TauCu]

Are you using the normal launcher? The worldedit-fabric:run_client config also seems to do it.
I'm running Windows 10 LTSC, builtin JRE for the launcher, Temurin-25.0.3+9 for Intellij/Environment.

This is the crash: https://pastebin.com/9kgejvG0
This is a video of the crash: enc.webm

When using fix/sessionmgr_thread_leak branch there is no crash.

I also did some more testing and it seems the executor service doesn't seem to cause a crash, but the timer is. Either way, they should be shutdown cleanly.