feat(examples): add Midscene Test Runner demo

[xiong35]

Summary

• Add a standalone midscene-test-runner-demo that demonstrates how to build and run a Web Test Project with @midscene/test.
• Use TodoMVC to show the boundary between declarative YAML cases and reusable TypeScript Nodes:
  • Playwright preset Nodes handle navigation and viewport setup.
  • Custom todo.* Nodes prepare data, capture state, and perform exact deterministic assertions.
  • Midscene Nodes use aiAct and aiAssert for visual interaction and user-facing semantic validation.
• Demonstrate Project setup and teardown, workflow lifecycle hooks, Zod-validated Node inputs, structured Node results, and Midscene reports.
• Add and commit the generated midscene-nodes.md reference so users can inspect the registered Node contract directly.
• Pin @midscene/test and @midscene/web to 1.12.3, add setup and troubleshooting documentation, and expose the demo from the repository README.

Test plan

• Run pnpm run typecheck.
• Run pnpm run describe:nodes and confirm it completes without Node warnings.
• Confirm the generated midscene-nodes.md exactly matches the current describe-nodes output.
• Run pnpm run browser:install.
• Run pnpm test without model configuration and verify:
  • the deterministic TodoMVC case passes in a real Chromium browser;
  • beforeEach, afterEach, and afterAll execute correctly;
  • the AI case reports the missing model configuration explicitly.