TestCode classification breaks in LSP for lines that begin with whitespace

[JoeRobich]

When using a raw string literal, lines that begin with whitespace are rendered all white.

// lang=c#-test
var code = """
    using System;

    namespace Test;

    public class Program
    {
        public static void Main(string[] args)
        {
            return;
        }
    }
    public struct Person {}
    public interface ICan {}
    """;

Test.Code.Bug.mp4

Noticed while working on #74433