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/collab_ui/src/chat_panel/message_editor.rs

@@ -454,8 +454,7 @@ impl MessageEditor {
         mut cx: AsyncWindowContext,
     ) {
         let (buffer, ranges) = cx
-            .background_executor()
-            .spawn(async move {
+            .background_spawn(async move {
                 let ranges = MENTIONS_SEARCH.search(&buffer, None).await;
                 (buffer, ranges)
             })