feat: 完善验证器和单元测试 (Task 3.1)

实现 Task 3.1 - C# 编译验证:
- CSharpCompilerValidator: C# 编译验证器
- AutoFixEngine: 3 轮自动修复引擎
- ValidationPipeline: 验证流水线服务
- 集成到 ConversionService

功能：
- 使用 Roslyn 进行实时编译验证
- 捕获编译错误和警告
- 自动修复第 1 轮：添加缺失的 using 语句
- 自动修复第 2 轮：类型映射修复
- 支持 1-3 轮验证迭代

测试覆盖 (新增 9 个测试)：
- CSharpCompilerValidatorTests: 3 个测试
- AutoFixEngineTests: 3 个测试
- ValidationPipelineTests: 3 个测试

总测试数：26 个，全部通过 ✅
Co-authored-by: monkeycode-ai <monkeycode-ai@chaitin.com>

CodePlay.Core/Services/ConversionService.cs

@@ -3,6 +3,7 @@ using CodePlay.Core.Models;
 using CodePlay.Core.Common;
 using CodePlay.Core.Parsers;
 using CodePlay.Core.Converters;
+using CodePlay.Core.Validators;
 
 namespace CodePlay.Core.Services;
 
@@ -12,12 +13,16 @@ namespace CodePlay.Core.Services;
 public class ConversionService
 {
     private readonly Dictionary<(LanguageType, LanguageType), IConverter> _converters = new();
+    private readonly ValidationPipeline _validationPipeline;
     
     public ConversionService()
     {
         // 注册转换器
         RegisterConverter(LanguageType.CSharp, LanguageType.Java, new CSharpToJavaConverter());
         RegisterConverter(LanguageType.Java, LanguageType.CSharp, new JavaToCSharpConverter());
+        
+        // 初始化验证流水线
+        _validationPipeline = new ValidationPipeline();
     }
     
     private void RegisterConverter(LanguageType source, LanguageType target, IConverter converter)
@@ -59,6 +64,24 @@ public class ConversionService
             // 执行转换
             var result = await converter.ConvertAsync(syntaxTree, request.TargetLanguage, request.Options, cancellationToken);
             
+            // 执行验证（仅当目标语言是 C# 时）
+            if (result.Success && request.TargetLanguage == LanguageType.CSharp && request.ValidationRounds > 0)
+            {
+                var validationSummary = await _validationPipeline.ValidateAsync(
+                    result.TransformedCode, 
+                    request.TargetLanguage, 
+                    request.ValidationRounds,
+                    cancellationToken);
+                
+                result.ValidationSummary = validationSummary;
+                
+                if (!validationSummary.Passed)
+                {
+                    result.Success = false;
+                    result.ErrorMessage = $"Validation failed after {validationSummary.RoundsExecuted} rounds";
+                }
+            }
+            
             return result;
         }
         catch (Exception ex)