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

Release Notes:

- N/A

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -101,7 +101,7 @@ impl MessageEditor {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> Self {
-        let this = cx.model().downgrade();
+        let this = cx.entity().downgrade();
         editor.update(cx, |editor, cx| {
             editor.set_soft_wrap_mode(SoftWrap::EditorWidth, cx);
             editor.set_use_autoclose(false);