LinqHeadTailAdderWithLimit.cs

internal class LinqHeadTailAdder : IIntegerAdder
{

    private const int maxSupportedLength = 10000;

    public int Add(int[] values)
    {
        if (values.Length > maxSupportedLength)
        {
            throw new NotSupportedException($"Max supported length is {maxSupportedLength}");
        }
        var (head, tail) = GetHeadAndTail(values);
        return AddImplentation(head, tail);
    }

    private static int AddImplentation(int head, IEnumerable<int> tail)
    {
        if (!tail.Any())
        {
            return head;
        }
        var (nextHead, nextTail) = GetHeadAndTail(tail);
        return AddImplentation(head + nextHead, nextTail);
    }

    private static (int head, IEnumerable<int> tail) GetHeadAndTail(IEnumerable<int> values) 
        => (values.First(), values.Skip(1));
    
}