project: Compare WorktreePaths by pairing, not by path sets (#62144)

# Objective

- Prevent changed main/folder associations from being skipped during
thread metadata updates.

## Solution

- Compare WorktreePaths as ordered main/folder pairs.
- Keep PathList equality order-insensitive.

## Testing

- cargo test -p project test_worktree_paths_equality_compares_pairings

## Self-Review Checklist:

- [x] I have reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed UI standards
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A