Add Playwright e2e hello-world test with CI job

[PatelUtkarsh]

No linked issue. Scaffolds the foundation for e2e testing so future PRs can build on it.

Approach

• Adopt @wordpress/e2e-test-utils-playwright (same utility Gutenberg uses) to keep the testing surface familiar and well-supported.
• Add a minimal tests/e2e/ suite:
  • playwright.config.js: chromium project, baseURL=http://localhost:8889 (wp-env tests site), retries/traces on CI, artifacts written under artifacts/.
  • global-setup.js: uses RequestUtils to authenticate the default admin / password wp-env user once and persist the auth cookies as Playwright storageState, so every test starts logged in.
  • hello-world.spec.js: two sanity checks. The front page responds with a non-empty <title>, and admin.visitAdminPage('index.php') shows the admin bar (confirms the login/storage-state path actually works).
• package.json: add @playwright/test and @wordpress/e2e-test-utils-playwright devDeps, plus test:e2e / test:e2e:debug scripts.
• .github/workflows/ci.yml: new dedicated e2e job (does not touch the existing build matrix). It runs checkout, Node 22, npm ci, npx playwright install --with-deps chromium, npm run build, npm run env:start (reuses the repo's existing .wp-env.json), npm run test:e2e, always stops wp-env, and uploads artifacts/ on failure.
• .gitignore: ignore local artifacts/, playwright-report/, test-results/.

QA notes

Local:

1. npm ci
2. npx playwright install chromium
3. npm run build
4. npm run env:start
5. npm run test:e2e, expect 2 passed.
6. npm run env:stop

Verify login actually happens (not just page-load): artifacts/storage-states/admin.json should exist and contain a wordpress_logged_in_* cookie after the run.

CI:

• Watch the new E2E (Playwright) job on this PR. It runs the same steps against wp-env on ubuntu-latest. On failure, Playwright traces/screenshots/videos are available under the playwright-artifacts workflow artifact.