[AI] Fix segfault on quit in neural restore gui_cleanup

[andriiryzhkov]

Fixes #20928

On quit, gui_cleanup flushed all pending events in the GLib main loop. One of them was a stale callback from another plugin already mid-teardown, and dispatching it crashed.

Fix

• Remove the flush – the idle callbacks it was meant to drain don't need it.
• Idle callbacks now skip safely if gui_cleanup already ran.
• Cancel all timers at the top of gui_cleanup so none can fire mid-cleanup.

Tested on macOS and Linux.

[TurboGit]

Looks good to me, thanks!