[PHPUnit120] Add MockObjectVarToStubRector

[TomasVotruba]

Adds MockObjectVarToStubRector — companion to PropertyCreateMockToCreateStubRector.

When a property native type is already \PHPUnit\Framework\MockObject\Stub, but its @var docblock still references MockObject, the docblock is updated.

/**
 * @var FieldModel|MockObject
 */
private \PHPUnit\Framework\MockObject\Stub $leadFieldModel;

becomes

/**
 * @var FieldModel|Stub
 */
private \PHPUnit\Framework\MockObject\Stub $leadFieldModel;

Handles union (|), intersection (&), and plain @var forms. Only fires when the native type is Stub, so real mock properties are untouched.

Registered in both the phpunit-code-quality and phpunit-mock-to-stub sets.