debugger: Align zoom behavior with other panels (#31901)

Release Notes:

- Debugger Beta: `shift-escape` (`workspace::ToggleZoom`) now zooms the
entire debug panel; `alt-shift-escape` (`debugger::ToggleExpandItem`)
triggers the old behavior of zooming a specific item.

crates/debugger_ui/src/debugger_ui.rs

@@ -49,6 +49,7 @@ actions!(
         ToggleThreadPicker,
         ToggleSessionPicker,
         RerunLastSession,
+        ToggleExpandItem,
     ]
 );