chore(fast-ssr): convert unit tests from Playwright to Node.js test runner

[radium-v]

Pull Request

📖 Description

Converts fast-ssr unit tests from Playwright to Node.js built-in test runner (node:test) and assertion module (node:assert/strict). The majority of tests are pure logic tests with no browser dependencies, so they don't need Playwright.

The single browser-dependent test (fast-style.pw.spec.ts) remains on Playwright and is separated by naming convention (*.pw.spec.ts).

👩‍💻 Reviewer Notes

• fast-style.spec.ts was renamed to fast-style.pw.spec.ts to distinguish it as a Playwright browser test from the pure Node tests.
• A cross-platform scripts/run-tests.mjs entry point is used to glob test files and invoke node --test, avoiding shell-specific glob expansion.
• The throws() calls in request-storage.spec.ts use { message: "..." } instead of a plain string to satisfy node:assert's API.
• Import order in directives.spec.ts was adjusted so the DOM shim loads before @microsoft/fast-element.

📑 Test Plan

All 191 Node-based tests pass with node run-tests.mjs. The Playwright-based fast-style.pw.spec.ts test continues to run via playwright test.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.