Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] 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

Closes #53116

Video 

[Screencast from 2026-04-04
19-14-55.webm](https://github.com/user-attachments/assets/d17945d6-b17c-435d-8155-648cd7ba574b)


Release Notes:

- Fixed: File duplication rename now selects the entire filename stem
instead of just the " copy" suffix, allowing users to type a new name
without manually clearing text

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