isHexadecimal function
Returns true if the value is a valid hexadecimal string
Implementation
bool isHexadecimal(String? value) {
if (value == null) return false;
final trimmed = value.trim();
if (trimmed.isEmpty) return false;
// Check if contains only hex characters (0-9, A-F, a-f)
final hexRegex = RegExp(r'^[0-9A-Fa-f]+$');
return hexRegex.hasMatch(trimmed);
}