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

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -724,7 +724,7 @@ fn handle_open_request(request: OpenRequest, app_state: Arc<AppState>, cx: &mut
 
                 let workspace_window =
                     workspace::get_any_active_workspace(app_state, cx.clone()).await?;
-                let workspace = workspace_window.root_model(&cx)?;
+                let workspace = workspace_window.entity(&cx)?;
 
                 let mut promises = Vec::new();
                 for (channel_id, heading) in request.open_channel_notes {