Remove internal CodeGenMember propeties from model factory method

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelFactoryProvider.cs

@@ -459,6 +459,13 @@ private static IReadOnlyList<ParameterProvider> GetParameters(
                     continue;
                 }
 
+                // Skip parameters for properties that were customized via CodeGenMember to be non-public
+                if (param.Property?.OriginalName != null
+                    && !param.Property!.Modifiers.HasFlag(MethodSignatureModifiers.Public))
+                {
+                    continue;
+                }
+
                 parameters.Add(GetModelFactoryParam(param));
             }
             return [.. parameters];

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/ModelFactoryProviderTests.cs

@@ -466,6 +466,36 @@ public void RequiredConstantPropertiesAreNotExposedAsParameters()
             Assert.AreEqual("return new global::Sample.Models.MockInputModel(\"constant\", prop2, additionalBinaryDataProperties: null);\n", factoryMethod.BodyStatements!.ToDisplayString());
         }
 
+        [Test]
+        public async Task SkipsCodeGenMemberInternalProperty()
+        {
+            var inputModel = InputFactory.Model(
+                "mockInputModel",
+                properties:
+                [
+                    InputFactory.Property("prop1", InputPrimitiveType.String),
+                    InputFactory.Property("prop2", InputPrimitiveType.String)
+                ]);
+
+            var mockGenerator = await MockHelpers.LoadMockGeneratorAsync(
+                inputModelTypes: [inputModel],
+                compilation: async () => await Helpers.GetCompilationFromDirectoryAsync());
+
+            var modelFactoryProvider = mockGenerator.Object.OutputLibrary.TypeProviders.OfType<ModelFactoryProvider>().SingleOrDefault();
+            Assert.IsNotNull(modelFactoryProvider);
+
+            var factoryMethod = modelFactoryProvider!.Methods.FirstOrDefault(m => m.Signature.Name == "MockInputModel");
+            Assert.IsNotNull(factoryMethod);
+
+            // prop1 is customized via CodeGenMember to be internal, so it should not appear in the factory method
+            Assert.AreEqual(1, factoryMethod!.Signature.Parameters.Count);
+            Assert.AreEqual("prop2", factoryMethod.Signature.Parameters[0].Name);
+
+            // The body should pass default for the internal property
+            var body = factoryMethod.BodyStatements!.ToDisplayString();
+            StringAssert.Contains("default", body);
+        }
+
         private static InputModelType[] GetTestModels()
         {
             InputType additionalPropertiesUnknown = InputPrimitiveType.Any;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/SkipsCodeGenMemberInternalProperty/MockInputModel.cs

@@ -0,0 +1,12 @@
+#nullable disable
+
+using Microsoft.TypeSpec.Generator.Customizations;
+
+namespace Sample.Models
+{
+    public partial class MockInputModel
+    {
+        [CodeGenMember("Prop1")]
+        internal string Prop1Internal { get; set; }
+    }
+}