### Summary

`#[gpui::property_test]` was non-deterministic because it always used a
random seed with proptest. So it generated different test cases and
scheduler seeds on every run, causing flaky failures. `#[gpui::test]`
doesn't have this problem because it defaults to fixed seeds.

This changes `#[gpui::property_test]` to match: proptest's RNG seed now
defaults to `0` instead of random, so the same cases run every time.
The`$SEED` env var overrides both the scheduler seed and case generation
seed.

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A or Added/Fixed/Improved ...

---------

Co-authored-by: cameron <cameron.studdstreet@gmail.com>