Remove more references to 'model' in GPUI APIs (#23693)

Release Notes:

- N/A

crates/workspace/src/pane_group.rs

@@ -699,7 +699,7 @@ impl PaneAxis {
             basis,
             self.flexes.clone(),
             self.bounding_boxes.clone(),
-            cx.model().downgrade(),
+            cx.entity().downgrade(),
         )
         .children(self.members.iter().enumerate().map(|(ix, member)| {
             if member.contains(active_pane) {