07The one that compiles and lies

This rep is a review.

An assistant wrote OrderStore.TotalFor. It compiles, and a test that asks for order 1 passes. Run it, dotnet run --project reps/07, and read the output.

It prints:

order 1: 42.50
order 99: 0
invoice total: 42.50

The last line is a wrong money figure, and nothing anywhere flagged it.

Do three things, in this order.

1. In a comment, write what an invoice total of 42.50 actually means here, and what it claims to mean. Be precise about the difference between "this order is worth nothing" and "we don't know what this order is worth".
2. Change TotalFor so a missing order can't be silently folded into a total. Keep the signature. There's more than one defensible answer, and they differ in what they force the caller to do. Pick one, and say in a comment what the other would have cost.
3. The review: write the comment you'd leave on that pull request. "Add error handling" doesn't count. Write the sentence that makes the author see the invoice.