isModelAvailable method
Check if model is available for current subscription
Implementation
Future<bool> isModelAvailable(String modelId) async {
  try {
    final userInfo = await getUserInfo();
    final subscription = userInfo['subscription'] as Map<String, dynamic>?;
    if (subscription == null) return false;
    final tier = subscription['tier'] as String?;
    final modelInfo = await getModelInfo(modelId);
    if (modelInfo == null) return false;
    // Basic availability check - in practice, this would depend on
    // the specific subscription tier and model requirements
    return tier != null;
  } catch (e) {
    return false;
  }
}