isJwtStructure function
Returns true if token looks like a JWT (three base64url parts separated by .).
Implementation
bool isJwtStructure(String token) {
final List<String> parts = token.split('.');
return parts.length == _jwtPartCount &&
parts.every((String p) => p.isNotEmpty && RegExp(r'^[A-Za-z0-9_-]+$').hasMatch(p));
}