Migrate tests from Karma to Vite + Vitest

[v-aidaba]

Karma and its Webpack test bundle are replaced by Vitest running in browser
mode on Playwright/Chromium, so the specs execute in a real browser without a
second build pipeline.

Testing

• Add vitest.config.mts and specs/tsconfig.json; delete karma.conf.ts and
  test.webpack.config.js
• Stub powerbi-visuals-api in specs/powerbiApiStub.ts so the ambient const
  enums survive the esbuild transform, and fail loudly on unstubbed members
• Replace Jasmine done callbacks with async/await built on the new
  TestWrapper.render() helper
• Swap Istanbul coverage for V8 coverage
• Remove obsolete specs for per-metric stale data settings, which the
  formatting model no longer supports

Scripts

• Add test:watch, test:coverage and test:typecheck
• Type check the specs before every test run
• Download Chromium from a dedicated browsers script (browsers:ci adds the
  Linux system packages) instead of a pretest hook, so test runs stay
  offline-friendly

CI

• Provision the browser in its own workflow step before running the tests
• Scope the audit to shipped dependencies (npm audit --audit-level=high
  --omit=dev)
• Update the GitHub actions to v6

Dependencies

• Drop the direct @typescript-eslint/parser and @typescript-eslint/eslint-plugin
  entries, which typescript-eslint already provides
• Move linting packages to devDependencies and drop the unused
  regenerator-runtime
• Update ESLint to v10, powerbi-visuals-tools to 7.2.1, powerbi-visuals-api to
  5.11.1 and the visual utils to their 7.x releases

No production code changed. Verified with lint, package, npm audit --omit=dev
and test:coverage (63/63 passing).