Multi instance setting ignored when default verb is parsed.

首先使用提供的 ParseArguments 和 MapResult 示例复现问题，尤其是 argTest5，并跟踪启用 AllowMultiInstance 后默认 verb 的解析方式。当默认 verb 接受重复的选项值而不报告该选项被定义多次，同时其他示例仍能正确解析时，即表示完成。

Describe the bug
When using the function "MapResult" to parse multiple verbs, the parser ignores the setting "AllowMultiInstance" while parsing the default verb.

Lib version: 2.9.1
Language: C#

To Reproduce
Test the following demo code to see the result.

Demo Class Options:

    [Verb("log", isDefault:true, HelpText = "Log command")]
    class LogOptions
    {
        [Option('a', HelpText = "Option A for log command")]
        public IEnumerable<string> OptionA { get; set; }

        [Option('b', HelpText = "Option B for log command")]
        public IEnumerable<string> OptionB { get; set; }
    }

    [Verb("save", HelpText = "Save command")]
    class SaveOptions
    {
        [Option('d', HelpText = "Option D for save command")]
        public IEnumerable<string> OptionD { get; set; }

        [Option('e', HelpText = "Option E for save command")]
        public IEnumerable<string> OptionE { get; set; }
    }

Demo code:

        Parser parser = new(x =>
        {
            x.HelpWriter = Console.Out;
            x.IgnoreUnknownArguments = true;
            x.AllowMultiInstance = true;
            x.AutoHelp = true;
        });

        var argTest1 = "log -a valueA -b valueB" .Split(' ');  // This works.
        var argTest2 = "save -d valueD -e valueE".Split(' ');  // This works.
        var argTest3 = "-a valueA -b valueB".Split(' ');       // This works.
        var argTest4 = "log -a valueA -b valueB -b valueB2".Split(' '); // This works.
        var argTest5 = " -a valueA -b valueB -b valueB2".Split(' ');     // This don't work.


        var res = parser.ParseArguments<LogOptions, SaveOptions>(argTest5).MapResult(
        (LogOptions opts) => ProcessLogCommand(opts),
        (SaveOptions opts) => ProcessSaveCommand(opts),
        errs => 1);
    }

When trying to parse argTest5 this error is shown:

  ERROR(S):
    Option 'b' is defined multiple times.
  
    -a           Option A for log command
  
    -b           Option B for log command
  
    --help       Display this help screen.
  
    --version    Display version information.

Expected behavior
Option "SaveOptions" correctly parsed with multiple arguments for argument b.