[CodeQuality] Skip inner function referenced as string callable in InnerFunctionToPrivateMethodRector

[TomasVotruba]

Fixes rectorphp/rector#9801

InnerFunctionToPrivateMethodRector only rewrites FuncCall references (inner()) when converting an inner function to a private method. If the inner function is referenced as a string callable (e.g. usort($items, 'sortByOrder')), that reference is a String_ node and is left untouched — the function gets moved into a private method while the call site still points at a now non-existent global function. With RemoveUnusedPrivateMethodRector also enabled, the "unused" method is then deleted entirely.

This PR skips the transformation when the inner function name is referenced as a string within the method, keeping working code intact.

Before

class Demo
{
    public static function getOptions()
    {
        function sortByOrder(array $a, array $b): int
        {
            return strcmp((string) $a['label'], (string) $b['label']);
        }

        usort($options, 'sortByOrder');
        return $options;
    }
}

was rewritten to broken code — sortByOrder() moved to a private method, but usort($options, 'sortByOrder') still referencing the removed global function.

After

No change — the inner function is left as-is, because it is referenced by string callable and cannot be safely rewritten.

[TomasVotruba]

LGTM 👍