Add "Fix with Assistant" code action on lines with diagnostics (#18163)

Release Notes:

- Added a new "Fix with Assistant" action on code with errors or
warnings.

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/project/src/project_tests.rs

@@ -2708,7 +2708,7 @@ async fn test_apply_code_actions_with_commands(cx: &mut gpui::TestAppContext) {
         .next()
         .await;
 
-    let action = actions.await[0].clone();
+    let action = actions.await.unwrap()[0].clone();
     let apply = project.update(cx, |project, cx| {
         project.apply_code_action(buffer.clone(), action, true, cx)
     });
@@ -5046,6 +5046,7 @@ async fn test_multiple_language_server_actions(cx: &mut gpui::TestAppContext) {
         vec!["TailwindServer code action", "TypeScriptServer code action"],
         code_actions_task
             .await
+            .unwrap()
             .into_iter()
             .map(|code_action| code_action.lsp_action.title)
             .sorted()