fix following bugs (#7406)

- Another broken following test
- Fix following between two windows

Release Notes:

- Fixed following when the leader has multiple Zed windows open

crates/workspace/src/item.rs

@@ -666,6 +666,7 @@ pub trait ProjectItem: Item {
         Self: Sized;
 }
 
+#[derive(Debug)]
 pub enum FollowEvent {
     Unfollow,
 }