Fix phpstan/phpstan#12871: Allow readonly property init in child classes on PHP 8.4+

[predictor2718]

Summary

Since PHP 8.4, readonly properties are implicitly protected(set), so a child class may initialize the parent's readonly property in its own constructor. PHPStan still reported property.readOnlyAssignOutOfClass for such assignments.

Changes

• src/Rules/Properties/ReadOnlyPropertyAssignRule.php: allow the "scope != declaring class" case when PhpVersion::supportsAsymmetricVisibility() is true, the property is not private(set), and the scope is a subclass of the declaring class. The existing constructor check still applies in the subclass's scope.
• tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php: inject PhpVersion, adjust existing testRule expectations for PHP 8.4+ (subclass-constructor writes to protected/public readonly are no longer reported; subclass non-constructor write now reports "outside of the constructor" instead of "outside of its declaring class").
• Added regression test testBug12871 with data file data/bug-12871.php covering the original snippet plus private(set) and non-constructor edge cases.

Test

vendor/bin/phpunit tests/PHPStan/Rules/Properties/ — 331 tests pass. Self-analysis [OK] No errors.

Fixes phpstan/phpstan#12871