Migrate unit test runner from Jest to Vitest

[Copilot]

This PR migrates the repository’s test infrastructure from Jest to Vitest to standardize on a modern ESM-native runner while preserving existing test behavior. The change is limited to test tooling and test entrypoints.

• Tooling migration

  • Replaced jest with vitest in devDependencies.
  • Updated the test script to run Vitest directly.

• Configuration cleanup

  • Removed obsolete jest.config.js now that Jest is no longer used.

• Test compatibility updates

  • Updated the test file to import test globals explicitly from Vitest (test, expect), matching current ESM usage.

{
  "scripts": {
    "test": "vitest run"
  },
  "devDependencies": {
    "vitest": "^4.1.9"
  }
}

import { expect, test } from "vitest";

[JoyceZhu]

@copilot update the whole Babel toolchain to v8.0.0 and above; js-yaml to v5.0.0 and above; markdown-it to 14.2.0 and above.

[Copilot]

@copilot update the whole Babel toolchain to v8.0.0 and above; js-yaml to v5.0.0 and above; markdown-it to 14.2.0 and above.

Addressed in a8b0394.

• Updated @github/markdownlint-github to ^0.8.2, which upgrades its transitive markdown-it to 14.2.0.
• js-yaml is at ^5.1.0.
• There are no @babel/* packages in the current lockfile/dependency graph after the Jest removal, so no Babel versions below 8 remain in resolved dependencies.