This change complements #38452 by adjusting `last_scheduled_scenario` to
account for reordered tasks in the inventory.

## Summary

Closes #58828
The "Rerun last debug scenario" action cycles through previous scenarios
instead of running the last one.

## Solution

Look for the last scenario from the front of the queue, as scenarios are
pushed to the front.

## Self-review

- [x] I've reviewed my diff for quality, security, reliability, and
performance.
- [x] UI changes follow the
[checklist](https://zed.dev/docs/development/ui-checklist).
- [ ] Tests cover the new or changed behavior.

Release Notes:

- debugger: Fixed "Rerun last debug scenario" not running the last
scheduled scenario.