tests-that-matterlisted

# Tests That Matter Prove behavior a caller or operator can observe. Green is not confidence. Match the repo's test runner and style. Do not invent a framework, coverage cult, or TDD ceremony. ## Choose scope - **Review:** findings only - no rewrite. Weak asserts, mock theater, missing failures/boundaries, false regressions. - **Write / fix:** add or reshape tests. Touch production only for a real test seam, and say so. - **New feature work:** when you change invariants or failure paths, update or add the focused cases that prove them. ## Prove - Public outcomes: return values, raised/returned errors, persisted state, and effects at real external boundaries. - Distinct failure modes as distinct cases (validation vs not-found vs infra vs partial success). If effect A commits and B fails, assert both the committed state and the failure. - Invariants and boundaries that can actually break (empty, zero, max, off-by-one, idempotency). - A regression that fails on the old defect. If it stays green on broken code, it is not a regression test. ## Avoid - Private helpers, call order inside the unit, and literal structure of the implementation. - Mocks of implementation details inside the unit. Prefer in-memory fakes for owned stateful collaborators; mock only narrow external or nondeterministic edges (network, clock, disk, queue, payment gateway). - Asserts that only restate the mock setup (`toHaveBeenCalledWith` with no outcome check). - Five near-duplicate cases when one tab