[Security] Fix RCE vulnerability in RestrictedUnpickler MRO check (#79057)

The _is_safe_class() function only checked the class's own __dict__ for
dangerous magic methods (__reduce__, __reduce_ex__, __getstate__, __setstate__).
It did NOT check the method resolution order (MRO), allowing classes that
inherit these methods from parent classes to bypass the safety check.

This fix checks all classes in the MRO (except object, whose default
__reduce__ is safe for user-defined classes) for dangerous method definitions.