Fix ArgumentException in OpenApiCSharpCodeGenerator.Sanitize method

Co-authored-by: christianhelle <710400+christianhelle@users.noreply.github.com>

Author: Copilot

src/Core/ApiClientCodeGen.Core.Tests/Generators/OpenApi/OpenApiCSharpCodeGeneratorSanitizeTests.cs

@@ -0,0 +1,27 @@
+using System.Reflection;
+using Rapicgen.Core.Generators.OpenApi;
+using Xunit;
+
+namespace ApiClientCodeGen.Core.Tests.Generators.OpenApi
+{
+    public class OpenApiCSharpCodeGeneratorSanitizeTests
+    {
+        [Fact]
+        public void Sanitize_Removes_SystemNetMime_Using_Statement()
+        {
+            // Arrange
+            const string codeWithMimeUsing = "using System;\nusing System.Net.Mime;\nnamespace Test {}";
+            const string expectedResult = "using System;\n\nnamespace Test {}";
+            
+            // Get the private Sanitize method through reflection
+            var sanitizeMethod = typeof(OpenApiCSharpCodeGenerator)
+                .GetMethod("Sanitize", BindingFlags.NonPublic | BindingFlags.Static);
+            
+            // Act
+            var result = sanitizeMethod?.Invoke(null, new object[] { codeWithMimeUsing }) as string;
+            
+            // Assert
+            Assert.Equal(expectedResult, result);
+        }
+    }
+}

src/Core/ApiClientCodeGen.Core/Generators/OpenApi/OpenApiCSharpCodeGenerator.cs

@@ -167,6 +167,6 @@ public string GenerateCode(IProgressReporter? pGenerateProgress)
         }
 
         private static string Sanitize(string code) =>
-            code.Replace("using System.Net.Mime;", null);
+            code.Replace("using System.Net.Mime;", "");
     }
 }