Modernize tests for PHPUnit 12 compatibility

Problem/Motivation

The Purge module's test suite uses patterns that are deprecated in PHPUnit 10/11 and removed in PHPUnit 12. This will cause test failures when Drupal core requires PHPUnit 12.

Steps to reproduce

Proposed resolution

Modernize all unit tests for PHPUnit 12 compatibility, eliminating all deprecation warnings.

Annotations → Attributes

• @dataProvider → #[DataProvider('methodName')]
• @group → #[Group('name')]
• @coversDefaultClass → #[CoversClass(ClassName::class)] or #[CoversTrait(TraitName::class)]
• Remove method-level @covers ::methodName (not supported as attributes)

Data Providers

• Change from protected function to public static function

Mock Methods

• ->will($this->returnValue($x)) → ->willReturn($x)
• ->will($this->returnValueMap([...])) → ->willReturnMap([...])

Trait Mocking

• Replace deprecated getMockForTrait() with anonymous class pattern

Interface Mocking

• Fix incorrect getMockForAbstractClass() on interface → createMock()

Code Quality

• Add docblocks to all test classes and methods
• Fix indentation inconsistencies

Remaining tasks

User interface changes

API changes

Data model changes