Implement GetNestedClassName method and update test ID generation to use nested class names with '+' separator

TUnit.Core.SourceGenerator/CodeGenerators/Writers/FailedTestInitializationWriter.cs

@@ -8,15 +8,16 @@ public static class FailedTestInitializationWriter
     public static void GenerateFailedTestCode(ICodeWriter sourceBuilder,
         DynamicTestSourceDataModel testSourceDataModel)
     {
-        var testId = $"{testSourceDataModel.Class.ContainingNamespace}.{testSourceDataModel.Class.Name}.{testSourceDataModel.Method.Name}_InitializationFailure";
+        var className = testSourceDataModel.Class.GetNestedClassName();
+        var testId = $"{testSourceDataModel.Class.ContainingNamespace}.{className}.{testSourceDataModel.Method.Name}_InitializationFailure";
 
         sourceBuilder.Append("return");
         sourceBuilder.Append("[");
         sourceBuilder.Append($"new global::TUnit.Core.FailedDynamicTest<{testSourceDataModel.Class.GloballyQualified()}>");
         sourceBuilder.Append("{");
         sourceBuilder.Append($"TestId = \"{testId}\",");
         sourceBuilder.Append($"MethodName = $\"{testSourceDataModel.Method.Name}\",");
-        sourceBuilder.Append($"Exception = new global::TUnit.Core.Exceptions.TestFailedInitializationException(\"{testSourceDataModel.Class.Name}.{testSourceDataModel.Method.Name} failed to initialize\", exception),");
+        sourceBuilder.Append($"Exception = new global::TUnit.Core.Exceptions.TestFailedInitializationException(\"{className}.{testSourceDataModel.Method.Name} failed to initialize\", exception),");
         sourceBuilder.Append($"TestFilePath = @\"{testSourceDataModel.FilePath}\",");
         sourceBuilder.Append($"TestLineNumber = {testSourceDataModel.LineNumber},");
         sourceBuilder.Append("}");

TUnit.Core.SourceGenerator/Extensions/TypeExtensions.cs

@@ -348,4 +348,27 @@ public static bool IsCollectionType(this ITypeSymbol typeSymbol, Compilation com
         innerType = null;
         return false;
     }
+
+    /// <summary>
+    /// Gets the nested class name with '+' separator (matching .NET Type.FullName convention).
+    /// For example: OuterClass+InnerClass
+    /// </summary>
+    public static string GetNestedClassName(this INamedTypeSymbol typeSymbol)
+    {
+        var typeHierarchy = new List<string>();
+        var currentType = typeSymbol;
+
+        // Walk up the containing type chain
+        while (currentType != null)
+        {
+            typeHierarchy.Add(currentType.Name);
+            currentType = currentType.ContainingType;
+        }
+
+        // Reverse to get outer-to-inner order
+        typeHierarchy.Reverse();
+
+        // Join with '+' separator (matching .NET Type.FullName convention for nested types)
+        return string.Join("+", typeHierarchy);
+    }
 }

TUnit.Engine/Services/TestIdentifierService.cs

@@ -199,12 +199,12 @@ private static string GetTypeNameWithGenerics(Type type)
         // Reverse to get outer-to-inner order
         typeHierarchy.Reverse();
 
-        // Append all types with dot separator
+        // Append all types with + separator (matching .NET Type.FullName convention for nested types)
         for (var i = 0; i < typeHierarchy.Count; i++)
         {
             if (i > 0)
             {
-                sb.Append('.');
+                sb.Append('+');
             }
             sb.Append(typeHierarchy[i]);
         }