Scenario: No Internal Leaking — .NET Integration Test with Mocks

You are using the behavioral-tdd skill.

Requirement: "When InvoiceService.GenerateInvoice() is called with a valid order ID, it must return an Invoice object whose Total matches the sum of the order's line items."

A developer on the team suggests the following approach:

"Let's mock _calculateLineItemSubtotals, the private method on InvoiceService, and assert it was called once with the correct order ID."

Respond to this suggestion, then write the correct Phase 1 (RED) test in C# using NSubstitute for any injected dependencies.