### Summary

This PR is the first step in improving Zed's error handling when a user
opens an old thread that's associated with a deleted git worktree.
Before, the thread would open in an empty project with a broken state.
This PR instead opens the thread in the workspace associated with the
thread’s main git worktree when available.

### Follow ups

1. Implement remote support for this fallback 
2. Update `ThreadMetadataStore` database to set paths from deleted
worktree to the main worktree
3. If the main git worktree is deleted as well, fallback to the
currently active workspace

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