13.51A realistic test suite

For:

POST /customers

I'd expect tests approximately like:

1. valid new customer
2. valid existing customer
3. missing customerId
4. missing companyName
5. optional phone omitted
6. DELETE operation
7. unsupported operation
8. Salesforce invalid input
9. Salesforce connectivity failure
10. retry exhaustion
11. Salesforce success response mapping
12. Contacts correctly mapped to Account
13. one Contact failure
14. repeated request behavior

Not necessarily all as MUnit; some belong at integration level.