Add integration tests for customers

[EduardoPires]

Summary

• update DatabaseConfig to support in-memory provider when running in the Testing environment
• expose Program as partial for test host discovery
• reference EFCore InMemory provider in the API project
• add xUnit integration test project using WebApplicationFactory
• test register, update and remove customer endpoints using AAA pattern

Testing

• dotnet test -c Release

---

https://chatgpt.com/codex/tasks/task_e_68478dcb81a8833395bde7fd3ef40531