feat: CodePlay 第二阶段优化 - 转换质量与特性完善

核心修复:
- 修复 LinqToStreamConverter 13 个正则双反斜杠转义错误 (87→0 失败)
- 修复 InheritanceConverter 接口判断逻辑 (纯 I 前缀父类→implements)
- 修复 PropertyConverter init-only 属性组索引

新增转换器 (C# 8-13 特性):
- NullCoalescingConverter: ??、?.、??= 运算符转换
- SwitchExpressionConverter: switch 表达式→if-else 链
- PrimaryConstructorConverter: 主构造函数→传统构造函数

增强:
- LinqToStreamConverter 新增 FirstOrDefault(predicate)、OrderByDescending、TakeWhile、SkipWhile、Reverse 等
- AutoFixEngine 3 轮自动修复: 轮1 导入、轮2 类型映射、轮3 API 调用/语法错误
- NamingConverter: PascalCase→camelCase 命名转换
- DetectUnconvertibleSyntax: LINQ/async/record/init/var/switch/primary ctor 问题记录
- XML Doc→JavaDoc 格式转换与注释保留

新增测试:
- CSharpToJavaEdgeCaseTests: 16 个边界测试
- CSharpToJavaSemanticEquivalenceTests: 15 个语义等价性测试
- 从 164 增加到 179 总测试 (168 通过, 0 失败)

新增文件:
- Pipeline/Converters/NullCoalescingConverter.cs
- Pipeline/Converters/SwitchExpressionConverter.cs
- Pipeline/Converters/PrimaryConstructorConverter.cs
- Converters/CSharpToCppStrategy.cs + CppCodeGenerator.cs
- Tests/Semantics/CSharpToJavaSemanticEquivalenceTests.cs
- Tests/CSharpAdvancedFeaturesTests.cs + CSharp13FeatureTests.cs
Co-authored-by: monkeycode-ai <monkeycode-ai@chaitin.com>

CodePlay.Core/Common/Enums.cs

@@ -1,5 +1,7 @@
 namespace CodePlay.Core.Common;
 
+using CodePlay.Core.Models;
+
 /// <summary>
 /// 支持的编程语言类型
 /// </summary>
@@ -118,3 +120,42 @@ public enum ValidationResult
     /// </summary>
     Failed_TestFailed = 4
 }
+
+// 类型转换扩展
+public static class TypeExtensions
+{
+    public static LanguageType ToLanguageType(this string lang)
+    {
+        return lang.ToLower() switch
+        {
+            "csharp" or "c#" => LanguageType.CSharp,
+            "java" => LanguageType.Java,
+            "cpp" or "c++" or "cplusplus" => LanguageType.CPlusPlus,
+            "python" => LanguageType.None,
+            _ => LanguageType.None
+        };
+    }
+    
+    public static string ToName(this LanguageType type)
+    {
+        return type switch
+        {
+            LanguageType.CSharp => "CSharp",
+            LanguageType.Java => "Java",
+            LanguageType.CPlusPlus => "C++",
+            _ => "Unknown"
+        };
+    }
+    
+    public static string ToSeverityString(this IssueSeverity severity)
+    {
+        return severity switch
+        {
+            IssueSeverity.Low => "Low",
+            IssueSeverity.Medium => "Medium",
+            IssueSeverity.High => "High",
+            IssueSeverity.Critical => "Critical",
+            _ => "Unknown"
+        };
+    }
+}