Use Playwright for integration tests

We currently use Selenium to test the examples in our documentation for errors. This is useful, but it doesn't catch features silently failing. This happened in our v0.19.0 release.

We need tests that actually verify the expected elements are visible on the page. This requires work: for each example we have to define what elements are expected, possibly after clicking on some other element.

I'd like to try to use Playwright for this. It looks pretty easy to use. It also integrates with Pytest. I'll give this a shot on a couple examples to see if this approach works well. If so, we can apply it to all examples.

まず、ドキュメントの例に対する既存の Selenium ベースの統合テストと、それらが Pytest でどのように実行されるかを確認します。いくつかの例で Playwright を試し、表示されることが期待される要素と、必要なクリックを定義します。サイレントに失敗する機能をこのアプローチで検出でき、すべての例に一貫して適用できれば完了です。