sum_tree: Replace rayon with futures (#41586)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Kate <kate@zed.dev>

crates/agent_ui/src/buffer_codegen.rs

@@ -487,9 +487,10 @@ impl CodegenAlternative {
     ) {
         let start_time = Instant::now();
         let snapshot = self.snapshot.clone();
-        let selected_text = snapshot
-            .text_for_range(self.range.start..self.range.end)
-            .collect::<Rope>();
+        let selected_text = Rope::from_iter(
+            snapshot.text_for_range(self.range.start..self.range.end),
+            cx.background_executor(),
+        );
 
         let selection_start = self.range.start.to_point(&snapshot);