Fix status mapping evaluation for non-persisted current status records

What does this MR do and why?

When a work item doesn't have a current status record (all work items created before the MVC1 release of configurable statuses in %18.2), they fall back to lifecycle's default status through a non-persisted CurrentStatus record using the status_with_fallback method.

This caused ArgumentError when find_mapped_status tried to compare nil updated_at with mapping's valid_until timestamp.

This MR sets updated_at to work item's created_at when building fallback current status records to support time-constrained status mapping evaluations.

This is consistent with existing filtering logic that compares mapping timestamps to issues.created_at.

Notes

• The issue started occurring in %18.6, after the deployment of !208414 (merged).
• This bug only affects the feature in %18.6, so no backports are required.

References

• #578030
• https://gitlab.com/gitlab-com/request-for-help/-/issues/3609

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a new group, project, and two issues.
2. Delete the current status record from the last issue via the Rails console: WorkItem.last.current_status.delete.
3. In the issues settings page (/groups/group-23-10/-/settings/issues), create a new status called New default.
4. Delete the existing To do status, map it to New default, and set New default as the default open status.
5. Re-add the To do status and make it the default again.
6. Navigate to the group or project work items list.
7. Before the fix: Expect to see the 'Something went wrong' error when fetching work items.
8. After the fix: The work item list and drawer should load without issues.

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