mcous/vitest-when

**Project summary** `vitest-when` adds a lightweight, declarative API for configuring stub behaviors on Vitest mock functions, letting you specify exact argument matches (via `calledWith`) and attach precise return, promise‑resolve, throw, or side‑effect handlers (`thenReturn`, `thenResolve`, `thenThrow`, `thenDo`, etc.) without scattering expectations across test phases. It’s aimed at developers who want clearer, harder‑to‑misconfigure mock setups—especially in TypeScript projects—where stub configuration lives entirely in the “arrange” stage and is fully type‑checked.