Closes https://github.com/zed-industries/zed/issues/32199
https://github.com/zed-industries/zed/issues/32229
https://github.com/zed-industries/zed/issues/32230
https://github.com/zed-industries/zed/issues/32232

Release Notes:

- Fixed a bug where if the last tab was a pinned tab and it was dragged
to the right, resulting in a no-op, it would become unpinned
- Fixed a bug where a pinned tab dragged just to the right of the end of
the pinned tab region would become unpinned
- Fixed a bug where dragging a pinned tab from one pane to another
pane's pinned region could result in an existing pinned tab becoming
unpinned when `max_tabs` was reached
- Fixed a bug where moving an unpinned tab to the left, just to the end
of the pinned region, would cause the pinned tabs to become unpinned.