🔧 JS SDKs: migrate tests off Vitest

[Robdel12]

Summary

Migrate the JS SDK packages’ test suites off Vitest and onto Node’s built-in test runner (node:test), to match the CLI.

Context

• CLI tests already run via node --test (see root package.json test script).
• JS SDK packages under clients/ currently run vitest run and import from vitest in tests.

Packages

• clients/storybook (@vizzly-testing/storybook)
• clients/static-site (@vizzly-testing/static-site)
• clients/vitest (@vizzly-testing/vitest) is a Vitest integration; likely keep Vitest here unless there’s a clear benefit + no loss of coverage.

Work Items

• Replace Vitest runner scripts:
  • npm test -> node --test ...
  • npm test:watch -> node --test --watch ...
• Rename or adjust discovery:
  • either rename tests/**/*.spec.js -> tests/**/*.test.js, or pass explicit globs so Node finds tests.
• Port test APIs:
  • import { describe, it } from 'node:test'
  • assertions via node:assert/strict
  • replace vi.fn()/spies with node:test mock helpers (or lightweight stubs) following the CLI patterns.
• Remove Vitest config + deps from affected packages:
  • delete vitest.config.js
  • remove vitest from devDependencies
• Ensure CI stays green:
  • .github/workflows/ci.yml client jobs already run npm test; they should pass without Vitest.

Acceptance Criteria

• clients/storybook: npm test passes on Node 22/24 without vitest installed.
• clients/static-site: npm test passes on Node 22/24 without vitest installed.
• No vitest dependency/config remains in those packages.
• CI client jobs remain green without changing their high-level steps.