test: add GET /workitems/{id} assertion in AmlInvestigationResourceTest

[mdproctor]

Context

Spec specs/2026-05-13-layer2-casehub-work-design.md called for a GET /workitems/{complianceReviewTaskId} assertion in AmlInvestigationResourceTest to verify WorkItem fields (claimDeadline, candidateGroups) at the REST layer.

This was not added. WorkItem field verification is covered at the service level by WorkItemAmlInvestigationServiceTest, making the resource-level assertion redundant. Known spec deviation — deferred.

Acceptance Criteria

• AmlInvestigationResourceTest makes a GET /workitems/{complianceReviewTaskId} after the POST
• Asserts claimDeadline is within the 30-day window
• Asserts candidateGroups contains "compliance-officers"