Use stored thread metadata during reset when no root thread view exists,
so retries keep the original session id and title instead of starting a
new session.

Add a regression test for reconnecting after an initial custom agent
load failure.

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

Closes #ISSUE

Release Notes:

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

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>