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/slash_command.rs

@@ -4,7 +4,7 @@ pub use assistant_slash_command::SlashCommand;
 use assistant_slash_command::{AfterCompletion, SlashCommandLine, SlashCommandWorkingSet};
 use editor::{CompletionProvider, Editor};
 use fuzzy::{match_strings, StringMatchCandidate};
-use gpui::{App, Context, Entity, Task, WeakEntity, Window};
+use gpui::{App, AppContext as _, Context, Entity, Task, WeakEntity, Window};
 use language::{Anchor, Buffer, CompletionDocumentation, LanguageServerId, ToPoint};
 use parking_lot::Mutex;
 use project::CompletionIntent;
@@ -162,7 +162,7 @@ impl SlashCommandCompletionProvider {
             let editor = self.editor.clone();
             let workspace = self.workspace.clone();
             let arguments = arguments.to_vec();
-            cx.background_executor().spawn(async move {
+            cx.background_spawn(async move {
                 Ok(completions
                     .await?
                     .into_iter()