feat: add Playwright E2E testing scaffold

[Kallyan01]

Summary

Add a Playwright end-to-end testing scaffold running against a dedicated wp-env
test environment, completing the shared OneSearch test baseline alongside the
Jest unit suite from #141.

Changes

• playwright.config.ts (new): Playwright setup rooted at testDir: ./tests/e2e.
• .wp-env.test.json (new): an isolated test environment on port 8889 with
  WP_DEBUG on and its own debug-test.log, so E2E runs never touch the dev
  container or its database.
• E2E specs (new):
  • tests/e2e/plugin/activation.spec.ts — activates and deactivates the plugin.
  • tests/e2e/scaffold/plugin-preview.spec.ts — asserts the built plugin is
    present and activatable, and that the admin surface renders without a fatal.
• Scripts: test:e2e (wp-scripts test-playwright) and wp-env:test
  (wp-env --config=.wp-env.test.json).
• @wordpress/env ^10.39.0 → ^11.8.0: 10.x has no --config flag, so
  wp-env:test silently no-opped against the already-running dev container
  instead of starting a separate test environment. The bump is what makes the
  isolated test env actually work.
• Adds @playwright/test and pins @wordpress/e2e-test-utils-playwright to
  1.46.0 (including an override for @wordpress/scripts) to keep the
  Playwright utils version consistent.