Add Decoder class and refactor argument parsing in B64TextEncoding playground

                                              "  可选字节序：\n" + 

                                              "    --le：小端序\n" + 

                                              "    --be：大端序（默认）\n" +

                                              "  可选字节序标记：" +

                                              "    --bom：带字节序标记" +

                                              "  可选字节序标记：\n" +

                                              "    --bom：带字节序标记\n" +

                                              "    不填：不带字节序标记（默认）";

        public Task Run(string[] args)

        {

            if (Errors.ValidArgsLength(args, 1))

            {

                var text = args[0];

                var encoderArg = args.Length > 1 ? args[1].ToLower() : "--u8";

                var orderArg = args.Length > 2 ? args[2].ToLower() : "--be";

                var bomArg = args.Length > 3 ? args[3].ToLower() : string.Empty;

            if (!TryGetEncodingFromArgs(args, out var encoder, out var text))

                return Task.CompletedTask;

                if (Errors.ValidArrowsArgWithName(encoderArg, "Encoder", "--u8", "--u16") &&

                    Errors.ValidArrowsArgWithName(orderArg, "Order", "--le", "--be") &&

                    Errors.ValidArrowsArgWithName(bomArg, "BOM", "--bom", string.Empty))

                {

                    var isBigEndian = orderArg switch

                    {

                        "--be" => true,

                        _ => false

                    };

            // 正向：字符串 -> 字节 -> Base64

            AnsiConsole.MarkupLine("[green]Encoded:[/]");

            AnsiConsole.WriteLine(Convert.ToBase64String(encoder.GetBytes(text)));

            AnsiConsole.WriteLine();

                    var isBom = bomArg switch

            return Task.CompletedTask;

        }

    }

    public class Decoder : IPlayground

    {

                        "--bom" => true,

                        _ => false

                    };

        public string LaunchCmd { get; } = "atob";

        public string? Name { get; } = "Base64String To String";

        public string? Description { get; } = "atob [Base64String] <Encoder> <Order> <BOM>\n" +

                                              "  注意：解码时的编码器、字节序、BOM 参数必须与编码时完全一致，否则会乱码或报错。\n" +

                                              "  可选参数同 btoa 命令。";

        public Task Run(string[] args)

        {

            if (!TryGetEncodingFromArgs(args, out var encoder, out var base64Text))

                return Task.CompletedTask;

                    Encoding encoder = encoderArg switch

            try

            {

                        "--u16" => new UnicodeEncoding(isBigEndian, isBom),

                        _ => new UTF8Encoding(isBigEndian, isBom)

                    };

                // 反向：Base64 -> 字节 -> 字符串

                byte[] bytes = Convert.FromBase64String(base64Text);

                string decodedText = encoder.GetString(bytes);

                    AnsiConsole.WriteLine(Convert.ToBase64String(encoder.GetBytes(text)));

                AnsiConsole.MarkupLine("[green]Decoded:[/]");

                // 使用 Markup 转义输出，防止解码出的字符串包含 Spectre.Console 的 [ ] 标记导致颜色错乱

                AnsiConsole.MarkupLine(Markup.Escape(decodedText)); 

                AnsiConsole.WriteLine();

            }

            catch (FormatException)

            {

                AnsiConsole.MarkupLine("[red]错误：输入的不是有效的 Base64 字符串。[/]\n");

            }

            catch (Exception ex)

            {

                AnsiConsole.MarkupLine($"[red]错误：解码失败 ({ex.Message})。请检查编码器/字节序参数是否与编码时一致。[/]\n");

            }

            return Task.CompletedTask;

        }

    }

    /// <summary>

    /// 提取的公共逻辑：解析参数并构建对应的 Encoding 对象

    /// </summary>

    private static bool TryGetEncodingFromArgs(string[] args, out Encoding encoder, out string payload)

    {

        encoder = Encoding.Default!; // 编译器要求初始化，实际会在下方被覆盖

        payload = string.Empty;

        if (!Errors.ValidArgsLength(args, 1))

            return false;

        payload = args[0];

        var encoderArg = args.Length > 1 ? args[1].ToLower() : "--u8";

        var orderArg = args.Length > 2 ? args[2].ToLower() : "--be";

        var bomArg = args.Length > 3 ? args[3].ToLower() : string.Empty;

        if (!Errors.ValidArrowsArgWithName(encoderArg, "Encoder", "--u8", "--u16") ||

            !Errors.ValidArrowsArgWithName(orderArg, "Order", "--le", "--be") ||

            !Errors.ValidArrowsArgWithName(bomArg, "BOM", "--bom", string.Empty))

        {

            return false;

        }

        bool isBigEndian = orderArg == "--be";

        bool isBom = bomArg == "--bom";

        encoder = encoderArg switch

        {

            "--u16" => new UnicodeEncoding(isBigEndian, isBom),

            _ => new UTF8Encoding(isBigEndian, isBom) // 注意：UTF-8 标准不区分大小端，这里传参仅为了保持签名一致

        };

        return true;

    }

}

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