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/fetch_command.rs

@@ -151,7 +151,7 @@ impl SlashCommand for FetchSlashCommand {
         let http_client = workspace.read(cx).client().http_client();
         let url = argument.to_string();
 
-        let text = cx.background_executor().spawn({
+        let text = cx.background_spawn({
             let url = url.clone();
             async move { Self::build_message(http_client, &url).await }
         });