debugger: Fix deadlock in on_app_quit with debugger running (#29372)

This fixes a deadlock that would occur when `DapStore` had its on quit
handler called. The deadlock was caused by `DapStore` spawning on the
main thread while `App::shutdown` blocks the main thread.

We added a debug_panic in GPUI that panics if a foreground task is
spawned while the App context is shutting down. This will help tests
catch hangs in `cx.on_app_quit` calls.

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/project/src/debugger/dap_store.rs

@@ -134,8 +134,6 @@ impl DapStore {
         breakpoint_store: Entity<BreakpointStore>,
         cx: &mut Context<Self>,
     ) -> Self {
-        cx.on_app_quit(Self::shutdown_sessions).detach();
-
         let locators = HashMap::from_iter([(
             "cargo".to_string(),
             Arc::new(super::locators::cargo::CargoLocator {}) as _,