Log when migrating thread metadata in `ThreadMetadataStore` (#52230)

## Context

This will make it easier to diagnose issues if any come up when
migrating thread metadata for the sidebar

## How to Review

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## Self-Review Checklist

<!-- Check before requesting review: -->
- [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

Release Notes:

- N/A
54f12d1ae1Bennet Bo Fenner committed on 3/23/2026, 4:57:12 PM· committed by GitHubparent60a2985
1 file changedLine totals unavailable