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/debugger_ui/src/tests/module_list.rs

@@ -214,12 +214,4 @@ async fn test_module_list(executor: BackgroundExecutor, cx: &mut TestAppContext)
         assert_eq!(actual_modules.len(), 2);
         assert!(!actual_modules.contains(&changed_module));
     });
-
-    let shutdown_session = project.update(cx, |project, cx| {
-        project.dap_store().update(cx, |dap_store, cx| {
-            dap_store.shutdown_session(session.read(cx).session_id(), cx)
-        })
-    });
-
-    shutdown_session.await.unwrap();
 }