Fix agent panel scroll while editing a past message (#30394)

This is similar to the `block_mouse_down` method added in #20649 (which
has a very similar motivation), but is more comprehensive in stopping
mouse events. Since I want to cherry-pick this to the releases, keeping
this change just to the agent panel. In a follow-up will replace
existing use of `block_mouse_down` to instead use this.

Release Notes:

- N/A

crates/agent/src/active_thread.rs

@@ -2235,7 +2235,7 @@ impl ActiveThread {
                 // Backdrop to dim out the whole thread below the editing user message
                 parent.relative().child(
                     div()
-                        .occlude()
+                        .stop_mouse_events_except_scroll()
                         .absolute()
                         .inset_0()
                         .size_full()