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/Services/UnconvertibleSyntaxHandler.cs

@@ -69,13 +69,13 @@ public class UnconvertibleSyntaxHandler
                 {
                     issues.Add(new ConversionIssue
                     {
-                        Type = IssueType.UnconvertibleSyntax,
-                        Severity = IssueSeverity.Medium,
+                        Type = IssueType.UnconvertibleSyntax.ToString(),
+                        Severity = IssueSeverity.Medium.ToString(),
                         LineNumber = lineNumber,
                         Description = $"C# keyword '{keyword}' cannot be directly converted to {targetLanguage}",
                         Suggestion = GetSuggestion(keyword, targetLanguage),
                         OriginalCode = line.Trim(),
-                        Language = sourceLanguage
+                        Language = sourceLanguage.ToName()
                     });
                 }
             }
@@ -87,26 +87,26 @@ public class UnconvertibleSyntaxHandler
                 {
                     issues.Add(new ConversionIssue
                     {
-                        Type = IssueType.UnconvertibleSyntax,
-                        Severity = IssueSeverity.Medium,
+                        Type = IssueType.UnconvertibleSyntax.ToString(),
+                        Severity = IssueSeverity.Medium.ToString(),
                         LineNumber = lineNumber,
                         Description = $"Pattern '{pattern}' cannot be directly converted to {targetLanguage}",
                         Suggestion = GetPatternSuggestion(pattern, targetLanguage),
                         OriginalCode = line.Trim(),
-                        Language = sourceLanguage
+                        Language = sourceLanguage.ToName()
                     });
                 }
                 else if (!pattern.StartsWith(@"\") && line.Contains(pattern))
                 {
                     issues.Add(new ConversionIssue
                     {
-                        Type = IssueType.UnconvertibleSyntax,
-                        Severity = IssueSeverity.Medium,
+                        Type = IssueType.UnconvertibleSyntax.ToString(),
+                        Severity = IssueSeverity.Medium.ToString(),
                         LineNumber = lineNumber,
                         Description = $"Pattern containing '{pattern}' cannot be directly converted to {targetLanguage}",
                         Suggestion = GetPatternSuggestion(pattern, targetLanguage),
                         OriginalCode = line.Trim(),
-                        Language = sourceLanguage
+                        Language = sourceLanguage.ToName()
                     });
                 }
             }
@@ -183,16 +183,16 @@ public class UnconvertibleSyntaxHandler
     {
         var issues = DetectUnconvertibleSyntax(sourceCode, sourceLanguage, targetLanguage);
         
-        var highSeverity = issues.Count(i => i.Severity == IssueSeverity.High);
-        var mediumSeverity = issues.Count(i => i.Severity == IssueSeverity.Medium);
-        var lowSeverity = issues.Count(i => i.Severity == IssueSeverity.Low);
+        var highSeverity = issues.Count(i => i.Severity == "High");
+        var mediumSeverity = issues.Count(i => i.Severity == "Medium");
+        var lowSeverity = issues.Count(i => i.Severity == "Low");
         
         var feasibility = new ConversionFeasibility
         {
             IsFeasible = highSeverity == 0 && mediumSeverity < 5,
             ConfidenceScore = CalculateConfidenceScore(issues),
             EstimatedManualEffort = CalculateEstimatedEffort(issues),
-            CriticalIssues = issues.Where(i => i.Severity == IssueSeverity.High).ToList(),
+            CriticalIssues = issues.Where(i => i.Severity == "High").ToList(),
             AllIssues = issues
         };
         
@@ -202,9 +202,9 @@ public class UnconvertibleSyntaxHandler
     private int CalculateConfidenceScore(List<ConversionIssue> issues)
     {
         var baseScore = 100;
-        baseScore -= issues.Count(i => i.Severity == IssueSeverity.High) * 20;
-        baseScore -= issues.Count(i => i.Severity == IssueSeverity.Medium) * 5;
-        baseScore -= issues.Count(i => i.Severity == IssueSeverity.Low) * 2;
+        baseScore -= issues.Count(i => i.Severity == "High") * 20;
+        baseScore -= issues.Count(i => i.Severity == "Medium") * 5;
+        baseScore -= issues.Count(i => i.Severity == "Low") * 2;
         
         return Math.Max(0, Math.Min(100, baseScore));
     }
@@ -213,9 +213,9 @@ public class UnconvertibleSyntaxHandler
     {
         var totalScore = issues.Sum(i => i.Severity switch
         {
-            IssueSeverity.High => 4,
-            IssueSeverity.Medium => 2,
-            IssueSeverity.Low => 1,
+            nameof(IssueSeverity.High) => 4,
+            nameof(IssueSeverity.Medium) => 2,
+            nameof(IssueSeverity.Low) => 1,
             _ => 1
         });