Fix the completions being too slow (#19013)

Closes https://github.com/zed-industries/zed/issues/19005

Release Notes:

- Fixed completion items inserted with a delay
([#19005](https://github.com/zed-industries/zed/issues/19005))

---------

Co-authored-by: Antonio Scandurra <antonio@zed.dev>

crates/copilot/src/copilot_completion_provider.rs

@@ -363,12 +363,10 @@ mod tests {
 
             // Confirming a completion inserts it and hides the context menu, without showing
             // the copilot suggestion afterwards.
-            editor.confirm_completion(&Default::default(), cx).unwrap()
-        })
-        .await
-        .unwrap();
-
-        cx.update_editor(|editor, cx| {
+            editor
+                .confirm_completion(&Default::default(), cx)
+                .unwrap()
+                .detach();
             assert!(!editor.context_menu_visible());
             assert!(!editor.has_active_inline_completion(cx));
             assert_eq!(editor.text(cx), "one.completion_a\ntwo\nthree\n");