Add wp-env integration tests for Handbook plugin

[dd32]

Summary

• Adds wp-env-based CI job for Handbook plugin integration tests using Docker
• Installs Yoast PHPUnit Polyfills in the wp-env test container (required by WP test suite)
• Updates Handbook test bootstrap:
  • Adds wp-env path fallback (/wordpress-phpunit/)
  • Auto-detects polyfills path from the WP test directory
• Adds void return types to setUp()/tearDown() for PHPUnit 9+ compatibility
• Updates sidebar assertion for WordPress show_in_rest key

Test plan

• All standalone PHP tests pass
• Handbook wp-env integration tests pass (89 tests)

🤖 Generated with Claude Code

[github-actions]

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