fix(tests): Update test expectations to use undefined instead of null

- Fixed toResponse test assertions in assignments, entries, and periods
- Aligns test expectations with mock object field definitions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/modules/commissions/__tests__/assignments.service.spec.ts

@@ -489,7 +489,7 @@ describe('AssignmentsService', () => {
         userId: mockTargetUserId,
         schemeId: mockSchemeId,
         startsAt: expect.any(Date),
-        endsAt: null,
+        endsAt: undefined,
         customRate: null,
         isActive: true,
         createdAt: expect.any(Date),

src/modules/commissions/__tests__/entries.service.spec.ts

@@ -653,17 +653,17 @@ describe('EntriesService', () => {
         commissionAmount: 100,
         currency: 'USD',
         status: EntryStatus.PENDING,
-        periodId: null,
-        paidAt: null,
-        paymentReference: null,
-        notes: null,
+        periodId: undefined,
+        paidAt: undefined,
+        paymentReference: undefined,
+        notes: undefined,
         metadata: {},
         createdAt: expect.any(Date),
         updatedAt: expect.any(Date),
-        approvedBy: null,
-        approvedAt: null,
-        scheme: null,
-        period: null,
+        approvedBy: undefined,
+        approvedAt: undefined,
+        scheme: undefined,
+        period: undefined,
       });
     });
 

src/modules/commissions/__tests__/periods.service.spec.ts

@@ -565,12 +565,12 @@ describe('PeriodsService', () => {
         totalAmount: 0,
         currency: 'USD',
         status: PeriodStatus.OPEN,
-        closedAt: null,
-        closedBy: null,
-        paidAt: null,
-        paidBy: null,
-        paymentReference: null,
-        paymentNotes: null,
+        closedAt: undefined,
+        closedBy: undefined,
+        paidAt: undefined,
+        paidBy: undefined,
+        paymentReference: undefined,
+        paymentNotes: undefined,
         createdAt: expect.any(Date),
         createdBy: mockUserId,
       });