Migrate test suites from Jest to Vitest

[nojibe]

Motivation

The project runs two separate Jest setups and the ESM story is painful. Migrating to Vitest gives us native ESM/TS support (no ts-jest, no --experimental-vm-modules), faster runs, a Jest-compatible API (low-friction migration), and unified web + node configs via a single vitest.workspace.

This ticket blocks #33 — CI should run the Vitest suites, not Jest, so this migration should land first.

Current state

• Web suite (jest.config.js): next/jest, jsdom, @testing-library/react, setup in _jest.setup.web.ts (mocks next/server, polyfills URLPattern). Ignores src/cli/ and src/lib/.
• CLI suite (jest.config.cli.js): ts-jest, node env, maxWorkers: 4, coverage, matches src/cli/**/*.test.ts and src/lib/**/*.test.ts.
• Setup: _jest.setup.web.ts, _jest.setup.cli.ts (global.jest = jest).
• ~95 test files. Heaviest areas: src/point-functions/__tests__ (24), src/lib/__tests__ (14), src/cli/services/__tests__ (13).
• Deps: jest, @jest/globals, @types/jest, jest-environment-jsdom, ts-jest.
• Scripts: test:web, test:cli, update-snapshots; test runs both.

Scope / plan

1. Add vitest, @vitest/coverage-v8, jsdom, @testing-library/jest-dom (or vitest-compatible matchers); remove jest, ts-jest, @jest/globals, @types/jest, jest-environment-jsdom.
2. Create a vitest.workspace.ts with two projects:
   • web: environment: 'jsdom', setup file port of _jest.setup.web.ts, @/ alias → src/, includes the current web globs.
   • cli/node: environment: 'node', includes src/cli/** and src/lib/**, coverage (v8) → coverage/cli.
3. Alias ^@/(.*)$ → src/$1 (via vite-tsconfig-paths or resolve.alias).
4. Port setup files: replace jest.mock('next/server', …) with vi.mock; keep the URLPattern polyfill; drop the global.jest = jest shim.
5. Codemod test files: jest.fn/mock/spyOn → vi.*, @jest/globals imports → vitest. Enable globals: true to minimize churn, or add explicit imports.
6. Verify snapshots migrate (Vitest uses a compatible format; regenerate with --update where needed).
7. Confirm aws-sdk-client-mock/sinon and next/server mocking still work under Vitest.
8. Update package.json scripts: test:web → vitest run --project web, test:cli → vitest run --project cli, test → vitest run, update-snapshots → vitest run --update; drop the --experimental-vm-modules flag.
9. Update CONTRIBUTING.md/docs test commands and remove Jest config files.

Acceptance criteria

• All existing tests pass under Vitest (both web and cli/node projects)
• pnpm test, pnpm test:web, pnpm test:cli run via Vitest; no --experimental-vm-modules
• Jest and ts-jest removed from package.json and lockfile
• Coverage still produced for the CLI suite
• Docs updated to reference Vitest
• No net loss in test coverage vs. the Jest run

Notes

Keep the two-project split (web jsdom vs. node) — the CLI/lib tests should not load the jsdom/Next setup. next/jest's SWC transform is replaced by Vitest's esbuild/SWC; watch for any Next-specific transform behavior (e.g. CSS module or next/font imports) in component tests.