Configures and runs Vitest - Vite-native unit framework with Jest-compatible API (`expect`, `vi.fn`, `vi.mock`, `vi.spyOn`); reads `vite.config.*` so existing Vite plugins work; supports in-source testing via `if (import.meta.vitest)`, browser-mode UI for headed tests, type-checking via `vitest --typecheck`, native ESM, and coverage via v8 (default) or istanbul providers. Use when the user works with Vite-based projects (Vue, Svelte, Solid, modern React with Vite) or is migrating from Jest on an existing Vite project (not bundler-free Node - use jest-tests for that).