Design for testability (unit tests, integration tests)?

Short answer: Write loosely coupled code via DI. Mock external dependencies for unit tests. Use in-memory databases (e.g., InMemory EF Core) for integration tests. Isolate layers to enable focused testing.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.