Fix issue in `StreamingEditFileTool` with incomplete last line (#51747)

<img width="927" height="368" alt="image"
src="https://github.com/user-attachments/assets/05de37f7-3034-4060-96b6-49c426170f39"
/>

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- N/A

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Co-authored-by: Antonio Scandurra <me@as-cii.com>
4e83c75681Bennet Bo Fenner committed on 3/26/2026, 9:53:49 AM· committed by GitHubparentc3d1f79
2 files changedLine totals unavailable