Replace 4 separate static function pointers (LogHandler, ErrorHandler,
ContentChangeHandler, ContentLoadedHandler) with a single MMKVHandler
abstract class in Core C++. Update all 6 platform wrappers (iOS, Android,
OHOS, Python, Golang, Flutter) to use adapter classes bridging to the new
unified handler. Add onMMKVContentLoadSuccessfully callback to Android,
OHOS, Flutter, Python and Golang where it was previously missing.