mask method
Masks the provided input using the strategy configuration.
Implementation
String mask(String input) {
if (input.isEmpty) return '';
final total = input.length;
final visible = keepStart + keepEnd;
final maskedCount = (total - visible).clamp(minMasked, total);
if (maskedCount <= 0) return maskChar * total;
final start = input.substring(0, keepStart.clamp(0, total));
final end = input.substring(total - keepEnd.clamp(0, total));
return '$start${maskChar * maskedCount}$end';
}