Actually run the eval and fix a hang when retrieving outline (#28547)

Release Notes:

- Fixed a regression that caused the agent to hang sometimes.

---------

Co-authored-by: Thomas Mickley-Doyle <tmickleydoyle@gmail.com>
Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/gpui/src/app/async_context.rs

@@ -1,7 +1,7 @@
 use crate::{
     AnyView, AnyWindowHandle, App, AppCell, AppContext, BackgroundExecutor, BorrowAppContext,
-    Entity, Focusable, ForegroundExecutor, Global, PromptLevel, Render, Reservation, Result, Task,
-    VisualContext, Window, WindowHandle,
+    Entity, EventEmitter, Focusable, ForegroundExecutor, Global, PromptLevel, Render, Reservation,
+    Result, Subscription, Task, VisualContext, Window, WindowHandle,
 };
 use anyhow::{Context as _, anyhow};
 use derive_more::{Deref, DerefMut};
@@ -154,6 +154,26 @@ impl AsyncApp {
         Ok(lock.update(f))
     }
 
+    /// Arrange for the given callback to be invoked whenever the given entity emits an event of a given type.
+    /// The callback is provided a handle to the emitting entity and a reference to the emitted event.
+    pub fn subscribe<T, Event>(
+        &mut self,
+        entity: &Entity<T>,
+        mut on_event: impl FnMut(Entity<T>, &Event, &mut App) + 'static,
+    ) -> Result<Subscription>
+    where
+        T: 'static + EventEmitter<Event>,
+        Event: 'static,
+    {
+        let app = self
+            .app
+            .upgrade()
+            .ok_or_else(|| anyhow!("app was released"))?;
+        let mut lock = app.borrow_mut();
+        let subscription = lock.subscribe(entity, on_event);
+        Ok(subscription)
+    }
+
     /// Open a window with the given options based on the root view returned by the given function.
     pub fn open_window<V>(
         &self,