gpui: Keep drag cursor style when dragging (#24797)

Release Notes:

- Improve to keep drag cursor style on dragging resize handles.

---

### Before


https://github.com/user-attachments/assets/d4100d01-ac02-42b8-b923-9f2b4633c458

### After


https://github.com/user-attachments/assets/b5a450cd-c6de-4b39-a79c-2d73fcbad209

With example:

```
cargo run -p gpui --example drag_drop
```


https://github.com/user-attachments/assets/4cba1966-1578-40ce-a435-64ec11bcace5

crates/gpui/src/window.rs

@@ -3093,6 +3093,7 @@ impl Window {
                             value: Arc::new(paths.clone()),
                             view: cx.new(|_| paths).into(),
                             cursor_offset: position,
+                            cursor_style: None,
                         });
                     }
                     PlatformInput::MouseMove(MouseMoveEvent {