[TASK] Create invoice operation helpers - Invoice-to-Payment Workflow

[africanitem]

Task Details

Epic: #512 [EPIC] Invoice-to-Payment Workflow E2E Tests

Dependencies

• Depends on: Epic [EPIC] Prerequisites and Test Data Setup Workflow E2E Tests #489

Success Criteria

• Helper functions in e2e/helpers/invoice-helpers.ts
• createTestInvoice() using pattern: INV-${Date.now()}
• postInvoice(), viewInvoice()
• Functions use Epic 0 helpers for customer/product creation