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/language_model_selector/src/language_model_selector.rs

@@ -228,8 +228,7 @@ impl PickerDelegate for LanguageModelPickerDelegate {
 
         cx.spawn_in(window, |this, mut cx| async move {
             let filtered_models = cx
-                .background_executor()
-                .spawn(async move {
+                .background_spawn(async move {
                     let displayed_models = if configured_providers.is_empty() {
                         all_models
                     } else {