Dropping a local Windows file (e.g. `E:\foo\bar.md`) onto a Zed window
connected to a WSL remote previously forwarded the path verbatim. On the
remote Linux side `E:\foo\bar.md` isn't absolute, so it got joined with
the worktree CWD, producing nonsense like `/home/user/E:\foo\bar.md` and
a "failed to canonicalize root path" error from the worktree scanner.

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

Closes #49915

before:
<img width="1343" height="331" alt="before"
src="https://github.com/user-attachments/assets/193cfbf6-d07e-4c5f-b941-75e4a0c62345"
/>

after:
<img width="1181" height="288" alt="after"
src="https://github.com/user-attachments/assets/7ad56e13-3723-4b48-aa3d-59eca6038a7f"
/>


Release Notes:

- Fixed dragging local Windows files onto a Zed window connected to a
WSL remote.