terminal: Fix pty info change detection race (#57257)

I think I introduced this at some point in the terminal agent situation.

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:

- N/A
43b7ca9cc2Ben Brandt committed on 5/20/2026, 1:13:00 PM· committed by GitHubparent1103bd8
1 file changedLine totals unavailable