Contract Test

Test what the two sides AGREED on, so either can change internals without breaking the other.

• Assert the shape: required fields, types, status codes, error format — not internal logic.
• Cover the contract's edges: missing optional fields, the documented error responses, pagination bounds, versioning.
• For consumers: test against the real contract (recorded/real responses), not a mock you wrote to match your assumptions — that mock drifts and lies.
• One source of truth for the schema; both sides validate against it. A green unit test with a wrong mock is worse than no test. Pin the contract.