Fixes a bug that caused duplicate sticky header rows to appear if
multiple outline items start on the same row.

Sort of addresses #52722, although arguably the real issue there is that
duplicate outline items are being created in the first place.

Before:


https://github.com/user-attachments/assets/7941cbe8-9b62-470c-b475-f08f2f20fac6

After:


https://github.com/user-attachments/assets/c4e291ea-6414-483f-8ff7-3d89d10000b6

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 that caused duplicate sticky header rows to appear if
multiple outline items start on the same row.