Closes [#46864](https://github.com/zed-industries/zed/issues/46864)
Initial PR: https://github.com/zed-industries/zed/pull/46573

## Summary

Fix a regression where dragging a tab onto another tab would place it at
the end of the tab bar instead of at the target position.

The issue was in the `on_drop` handler: it used `this.items.len()`
(evaluated at drop time) instead of the captured `ix` index (the
position of the tab being dropped onto).

Release Notes:

- Fixed tab reordering not working correctly when dragging tabs

## Video after fix:

- when 'show_pinned_tabs_in_separate_row' is false:


https://github.com/user-attachments/assets/1ede0ce5-1161-4209-83f4-33a07572782a

- when 'show_pinned_tabs_in_separate_row' is true:


https://github.com/user-attachments/assets/d56c0e59-8372-41d4-973b-32a4895ca729

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Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>