Fix terminal pane tabs arrangement and closing (#22013)

* Fixes the inability to drag and drop terminal tabs to reorder them;
fixed incorrect terminal tab move on drag and drop into existing pane
(follow-up of https://github.com/zed-industries/zed/pull/21238)
* Fixes save dialogue appearing when on closing terminal tabs with
running tasks (follow-up of
https://github.com/zed-industries/zed/pull/21374)

Release Notes:

- Fixed terminal pane tabs arrangement and closing