Refactor `ShipmentReviewService` to update the existing pending review
instead of creating a new `ShipmentReview` record during the approval
and rejection processes. This ensures that the review lifecycle
correctly finalizes the current pending revision.
- Implement `getPendingReview` to retrieve the active pending review
- Update `approve` to modify the existing review record
- Update `reject` to modify the existing review record
- Add documentation clarifying when new revisions are created versus
when existing ones are finalized
