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_commands/src/search_command.rs

@@ -119,8 +119,7 @@ impl SlashCommand for SearchSlashCommand {
             let loaded_results = SemanticDb::load_results(results, &fs, &cx).await?;
 
             let output = cx
-                .background_executor()
-                .spawn(async move {
+                .background_spawn(async move {
                     let mut text = format!("Search results for {query}:\n");
                     let mut sections = Vec::new();
                     for loaded_result in &loaded_results {