## Summary

Fixes crash when dragging debugger panel items
(terminal/console/variables/frames) to another pane as split view.

  **Root causes:**
1. Double borrow panic - `pane_group.split()` was called synchronously
inside a `Context<Pane>` update, causing "cannot update Pane while it is
already being updated"
2. `unwrap()` calls that panic when items are in transition during drag
operations
3. `debug_assert!` in `pane_at_pixel_position` fails when `members` and
`bounding_boxes` are temporarily out of sync after deferred split

  **Fixes:**
- Defer entire split+move operation via `cx.spawn_in()` to avoid double
borrow
- Replace `unwrap()` with graceful early returns in `activate_item` and
`run_in_terminal`
- Handle `members.len() != bounding_boxes.len()` gracefully in
`pane_at_pixel_position`

Closes #46784

Closes #ISSUE

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>