sum_tree: Replace rayon with futures (#41586)

Release Notes:

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

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

crates/channel/src/channel_buffer.rs

@@ -70,6 +70,7 @@ impl ChannelBuffer {
                 ReplicaId::new(response.replica_id as u16),
                 capability,
                 base_text,
+                cx.background_executor(),
             )
         })?;
         buffer.update(cx, |buffer, cx| buffer.apply_ops(operations, cx))?;