cx.background_executor().spawn(...) -> cx.background_spawn(...) (#25103)

Done automatically with

> ast-grep -p '$A.background_executor().spawn($B)' -r
'$A.background_spawn($B)' --update-all --globs "\!crates/gpui"

Followed by:

* `cargo fmt`
* Unexpected need to remove some trailing whitespace.
* Manually adding imports of `gpui::{AppContext as _}` which provides
`background_spawn`
* Added `AppContext as _` to existing use of `AppContext`

Release Notes:

- N/A

crates/assistant_context_editor/src/context.rs

@@ -849,7 +849,7 @@ impl AssistantContext {
             .collect::<Vec<_>>();
         context_ops.extend(self.pending_ops.iter().cloned());
 
-        cx.background_executor().spawn(async move {
+        cx.background_spawn(async move {
             let buffer_ops = buffer_ops.await;
             context_ops.sort_unstable_by_key(|op| op.timestamp());
             buffer_ops