Add comprehensive Behat testing guide to handbook

[Copilot]

Created comprehensive Behat tests documentation guide:

• Create new guide file guides/behat-tests.md with comprehensive Behat testing documentation
  • Introduction section: What are Behat tests, why they matter, getting started
  • Writing your first test: Basic structure, common steps, running tests
  • Advanced usage section: Effective tests, variables, structured output, error testing, custom steps
  • Best practices: Do's and don'ts for writing maintainable tests
  • Scaffolding and troubleshooting sections
  • Additional resources and examples
• Update guides.md to include link to new Behat tests guide
• Update index.md to include link to new Behat tests guide
• Update bin/handbook-manifest.json to register the new guide
• Update contributions/pull-requests.md to link to the new comprehensive guide
• Update guides/external-resources.md to add the Delicious Brains article
• Address review feedback: Add SQLite option and improve database setup documentation
  • Recommend SQLite as the easiest option (no database setup required)
  • Remove manual MySQL database creation steps
  • Emphasize using composer prepare-tests for MySQL setup
  • Update troubleshooting section to mention SQLite first

The guide now provides a clear, easy path for contributors to start testing with SQLite, while still documenting MySQL/MariaDB options.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create a dedicated doc for how to write Behat tests</issue_title>
<issue_description>WP-CLI integration test documentation exists in two places:

• https://make.wordpress.org/cli/handbook/pull-requests/#running-and-writing-tests
• https://github.com/wp-cli/scaffold-package-command#wp-scaffold-package-tests

It would be more user-friendly to have an uberdoc for Behat tests. We could have two sections:

1. Introduction - Getting started, and writing your first test
2. Advanced usage - Now that you've written a test, here's how you write steps, etc.

Once this document exists, we can have both existing places link directly to it.

Related https://deliciousbrains.com/writing-functional-tests-wp-cli-packages/</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create a dedicated doc for how to write Behat tests #3

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