docs: clarify dependency injection in PHP tests

[Epic-Drytu]

Description

Fixes #15296

This updates the PHP testing documentation to clarify how dependency injection can be used in tests.

It explains:

• when dependencies can be mocked or stubbed directly in unit tests
• when the dependency injection container is useful for integration tests
• how to access the application container in a test
• how mocked services can be replaced in the container

Validation

• sphinx-lint developer_manual/basics/testing.rst passes
• Full documentation build was not run successfully because this local checkout is a sparse checkout and is missing parts of the documentation tree

[github-actions]

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