This PR should fix the issue Ben ran into with Auto Watch. It isn’t a
bug in Auto Watch itself, but a bug in Zed’s active-pane bookkeeping
when Zed auto-closes a pane. Ben had a screen share in its own split;
when the share ended, the tab closed, and with it, the pane. In this
case, Zed was holding onto the removed pane as the active pane, so Auto
Watch was trying to open the next screen share in a pane that was no
longer part of the workspace.

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [X] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [X] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a bug where Zed incorrectly kept a removed pane as the active
pane.