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/docs_command.rs

@@ -176,7 +176,7 @@ impl SlashCommand for DocsSlashCommand {
             .provider()
             .ok_or_else(|| anyhow!("no docs provider specified"))
             .and_then(|provider| IndexedDocsStore::try_global(provider, cx));
-        cx.background_executor().spawn(async move {
+        cx.background_spawn(async move {
             fn build_completions(items: Vec<String>) -> Vec<ArgumentCompletion> {
                 items
                     .into_iter()
@@ -284,7 +284,7 @@ impl SlashCommand for DocsSlashCommand {
 
         let args = DocsSlashCommandArgs::parse(arguments);
         let executor = cx.background_executor().clone();
-        let task = cx.background_executor().spawn({
+        let task = cx.background_spawn({
             let store = args
                 .provider()
                 .ok_or_else(|| anyhow!("no docs provider specified"))