edit_prediction: Fix v4 cursor placement (#61901)

# Objective

Prevent accepted v4 edit predictions from moving the cursor past the
model's predicted position.

## Solution

Keep the cursor's base anchor fixed while applying inserted text, then
apply the model-provided offset. Add regression coverage based on the
reported cursor jump.

## 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
f851d82e88Ben Kunkle committed on 7/30/2026, 7:42:45 AM· committed by GitHubparentb9301f5
1 file changedLine totals unavailable