[DeadCode] Keep private method called via self::class static call

[TomasVotruba]

Why

RemoveUnusedPrivateMethodRector removes private methods that look unused to static analysis. But a private method invoked through a class-string static call is not seen by the usage analyzer, so it gets wrongly removed.

This pattern was found by scanning withSkip() across popular Rector configs - real projects disable the rule on files where private methods are called this way.

Code sample

final class SomeClass
{
    public function run(): object
    {
        return self::class::sampleClass();
    }

    private function sampleClass(): object
    {
        return new \stdClass();
    }
}

Before this PR, sampleClass() is removed (the self::class::sampleClass() call is invisible to the analyzer), breaking run(). After, it is kept.

Fix

resolveClassStringStaticCallNames() collects method names invoked via a ::class static call (self::class::x(), static::class::x(), Foo::class::x()) and skips removing them.

Tests

New keep-fixture keep_self_class_static_call.php.inc. ECS + PHPStan clean, all rule tests green.