[docs] Document new externallib_testcase

[andrewnicols]

Note: I have deliberately not updated the stable versions. This change has been applied to 4.4, 4.5, 5.0, and main. It has not been applied to 4.1.

[netlify]

✅ Deploy Preview for moodledevdocs ready!

Name	Link
🔨 Latest commit	95686f0
🔍 Latest deploy log	https://app.netlify.com/projects/moodledevdocs/deploys/689d547286fe640008488ddd
😎 Deploy Preview	https://deploy-preview-1419--moodledevdocs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[Copilot]

Pull Request Overview

This PR updates the external services testing documentation to reflect changes in the testing framework. The documentation now recommends using a new externallib_testcase class for testing external services and updates the example code to follow modern PHPUnit practices.

• Introduces documentation for the new \core_external\tests\externallib_testcase class
• Updates example test code to use modern PHPUnit attributes and practices
• Removes deprecated testing patterns and simplifies the test setup

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}