Add practical PHP testing guidance to AGENTS.md

[kraftbj]

Fixes #

Proposed changes

While working on the packages/connection package, an AI coding agent couldn't figure out how to run the package's PHP tests. It guessed commands like jp phpunit (which doesn't exist) and failed to discover the correct jp test php invocation with its flags. The existing AGENTS.md Testing section listed the basic commands but lacked the practical details agents need to actually run and write tests.

This expands the PHP Testing section with:

• Clarification that jp test php works for all projects except plugins/jetpack (which uses jp docker commands)
• --verbose and --filter flag usage with concrete examples
• WorDBless testing pattern: extend WorDBless\BaseTestCase, use wp_insert_user() + get_userdata() instead of self::factory()->user->create_and_get() (which is not available in WorDBless)
• Reference to an existing test example in projects/packages/connection/tests/php/sso/Helpers_Test.php

Other information

• Generate changelog entries for this PR (using AI).

Related product discussion/links

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Review the diff to AGENTS.md for accuracy and completeness.
• No functional code changes — documentation only.

[github-actions]

Thank you for your PR!

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