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_slash_command/src/extension_slash_command.rs

@@ -103,7 +103,7 @@ impl SlashCommand for ExtensionSlashCommand {
     ) -> Task<Result<Vec<ArgumentCompletion>>> {
         let command = self.command.clone();
         let arguments = arguments.to_owned();
-        cx.background_executor().spawn(async move {
+        cx.background_spawn(async move {
             let completions = self
                 .extension
                 .complete_slash_command_argument(command, arguments)
@@ -135,7 +135,7 @@ impl SlashCommand for ExtensionSlashCommand {
     ) -> Task<SlashCommandResult> {
         let command = self.command.clone();
         let arguments = arguments.to_owned();
-        let output = cx.background_executor().spawn(async move {
+        let output = cx.background_spawn(async move {
             let delegate =
                 delegate.map(|delegate| Arc::new(WorktreeDelegateAdapter(delegate.clone())) as _);
             let output = self