[WIP] Run end-to-end tests for Visualizer plugin

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

We just want to make sure you can run e2e tests. Please run e2e tests and confirm they all work. To run the E2E tests for the Visualizer plugin, from the plugin root directory run npm install to install dependencies, then npm run test:env:start to start the WordPress environment via Docker (this may take a minute on first run as it pulls images). Once the environment is up, run npm run test:e2e:playwright to execute the full test suite. To run a specific spec file, use npx wp-scripts test-playwright --config tests/e2e/playwright.config.js tests/e2e/specs/gutenberg-editor.spec.js. When finished, run npm run test:env:stop to shut down the environment, or npm run test:env:clean if you need a fresh database state before re-running.

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