edit_prediction: Prevent v4 cursor marker leaks (#61957)

# Objective

Prevent V4 cursor markers from appearing in edit prediction previews or
accepted edits.

## Solution

Replace the cursor marker with an internal sentinel before sub-line
diffing, so matching source text cannot reconstruct the marker.

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A
4f047acc17Ben Kunkle committed on 7/31/2026, 9:53:28 AM· committed by GitHubparentb209000
1 file changedLine totals unavailable