gpui: Actually remove the Result from AsyncApp (#45809)

Depends on: https://github.com/zed-industries/zed/pull/45768

Refactor plan:
https://gist.github.com/mikayla-maki/6c4bf263fd80050715ba01f45478796e
Overall plan:
https://gist.github.com/mikayla-maki/7bb5078e4385a2e683e1e1eb40d17d38

This is the big one.

Release Notes:

- N/A

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

crates/codestral/src/codestral.rs

@@ -283,7 +283,7 @@ impl EditPredictionDelegate for CodestralEditPredictionDelegate {
             let edits: Arc<[(Range<Anchor>, Arc<str>)]> =
                 vec![(cursor_position..cursor_position, completion_text.into())].into();
             let edit_preview = buffer
-                .read_with(cx, |buffer, cx| buffer.preview_edits(edits.clone(), cx))?
+                .read_with(cx, |buffer, cx| buffer.preview_edits(edits.clone(), cx))
                 .await;
 
             this.update(cx, |this, cx| {