fix(http-client-csharp): override MRW *Core methods when external base declares them (#11044)

Fixes #11042

## Problem
Models derived from an external/referenced MRW-generated base type (e.g.
`Azure.AI.Extensions.OpenAI` types deriving from OpenAI's
`ResponseItem`) generated `PersistableModelCreateCore` and the other
serialization `*Core` methods as `protected virtual` instead of
`protected override`. The compiler then reports CS0114 (`hides inherited
member ... add the override keyword`).

## Root cause
PR #11002 routes external `InputModelType`s through
`SystemObjectModelProvider` so derived models get a real `ModelProvider`
base. However
`SystemObjectModelProvider.ShouldSkipDerivedSerializationMethodOverrides`
unconditionally returns `true` (introduced in #9862 for hand-authored
ARM bases like `ResourceData` that do **not** declare the generated
`*Core` methods). Reusing it for *all* external bases forced derived
models to hide rather than override even when the external base is
itself MRW-generated and exposes the virtual `*Core` methods.

## Fix
`MrwSerializationTypeDefinition` now decides whether to skip the derived
overrides by reflecting on the wrapped framework type when the base is a
`SystemObjectModelProvider`: if it declares all four generated `*Core`
methods (`JsonModelWriteCore`, `JsonModelCreateCore`,
`PersistableModelWriteCore`, `PersistableModelCreateCore`), the derived
model emits `protected override`; otherwise existing behavior is
preserved (bases without those methods, e.g. `object`/`ResourceData`,
still emit `virtual`).

## Tests
Added regression tests in `SystemObjectModelSerializationTests` covering
an external base that follows the generated MRW pattern, asserting the
four `*Core` methods are emitted as `override`. Existing tests (object
base -> virtual) remain unchanged. Full ClientModel suite (1435 tests)
passes.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: JoshLove-msft

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

@@ -94,7 +94,7 @@ public MrwSerializationTypeDefinition(InputModelType inputModel, ModelProvider m
             _additionalBinaryDataProperty = new(GetAdditionalBinaryDataPropertiesProp);
             _additionalProperties = new(() => [.. _model.Properties.Where(p => p.IsAdditionalProperties)]);
             _shouldOverrideMethods = _model.BaseModelProvider != null && !_isStruct;
-            _shouldSkipDerivedSerializationMethodOverrides = _model.BaseModelProvider?.ShouldSkipDerivedSerializationMethodOverrides == true;
+            _shouldSkipDerivedSerializationMethodOverrides = ShouldSkipDerivedSerializationMethodOverrides(_model.BaseModelProvider);
             _utf8JsonWriterSnippet = _utf8JsonWriterParameter.As<Utf8JsonWriter>();
             _mrwOptionsParameterSnippet = _serializationOptionsParameter.As<ModelReaderWriterOptions>();
             _jsonElementParameterSnippet = _jsonElementDeserializationParam.As<JsonElement>();
@@ -157,6 +157,57 @@ private static bool IsModelType(CSharpType type)
             => ScmCodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(type, out var baseProvider) &&
                baseProvider is ModelProvider;
 
+        /// <summary>
+        /// Determines whether a derived model should skip overriding the generated serialization
+        /// <c>*Core</c> methods of its base.
+        /// </summary>
+        /// <remarks>
+        /// External/system base models are represented by a <see cref="SystemObjectModelProvider"/>.
+        /// Such a base only participates in the generated MRW <c>*Core</c> override chain when the
+        /// wrapped framework/referenced type itself follows the model-reader-writer serialization
+        /// pattern (i.e. implements <see cref="IJsonModel{T}"/> or <see cref="IPersistableModel{T}"/>,
+        /// and therefore exposes the overridable <c>*Core</c> methods). When it does, derived models
+        /// must override those methods rather than hide them (otherwise the compiler reports CS0114).
+        /// When it does not (for example a hand-authored base such as <c>ResourceData</c>), derived
+        /// models re-introduce the methods as <c>virtual</c>.
+        /// </remarks>
+        private static bool ShouldSkipDerivedSerializationMethodOverrides(ModelProvider? baseModelProvider)
+        {
+            if (baseModelProvider is null)
+            {
+                return false;
+            }
+
+            if (baseModelProvider is SystemObjectModelProvider systemBase)
+            {
+                return !SystemTypeImplementsModelReaderWriter(systemBase.SystemType);
+            }
+
+            return baseModelProvider.ShouldSkipDerivedSerializationMethodOverrides;
+        }
+
+        private static bool SystemTypeImplementsModelReaderWriter(CSharpType systemType)
+        {
+            if (!systemType.IsFrameworkType)
+            {
+                return false;
+            }
+
+            foreach (var @interface in systemType.FrameworkType.GetInterfaces())
+            {
+                if (@interface.IsGenericType)
+                {
+                    var definition = @interface.GetGenericTypeDefinition();
+                    if (definition == typeof(IJsonModel<>) || definition == typeof(IPersistableModel<>))
+                    {
+                        return true;
+                    }
+                }
+            }
+
+            return false;
+        }
+
         protected override ConstructorProvider[] BuildConstructors()
         {
             List<ConstructorProvider> constructors = new();

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/SystemObjectModelSerializationTests.cs

@@ -2,7 +2,9 @@
 // Licensed under the MIT License.
 
 using System;
+using System.ClientModel.Primitives;
 using System.Linq;
+using System.Text.Json;
 using Microsoft.TypeSpec.Generator.ClientModel.Providers;
 using Microsoft.TypeSpec.Generator.Input;
 using Microsoft.TypeSpec.Generator.Primitives;
@@ -201,5 +203,127 @@ public void JsonModelCreateCore_IsOverride_WhenBaseIsRegularModel()
             Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
                 "JsonModelCreateCore should be 'override' with regular base model");
         }
+
+        /// <summary>
+        /// Creates a derived model whose SystemObjectModelProvider base wraps a framework type that
+        /// itself follows the generated MRW pattern (declares the protected virtual *Core methods).
+        /// </summary>
+        private static (ModelProvider Model, MrwSerializationTypeDefinition Serialization) CreateDerivedModelWithMrwSystemBase()
+        {
+            var baseProp = InputFactory.Property("Name", InputPrimitiveType.String);
+            var baseInputModel = InputFactory.Model("Resource", properties: [baseProp]);
+            var derivedProp = InputFactory.Property("Location", InputPrimitiveType.String);
+            var derivedInputModel = InputFactory.Model("TrackedResource", properties: [derivedProp], baseModel: baseInputModel);
+
+            // The base wraps a framework type that declares the generated MRW *Core methods, so a
+            // derived model must override them (mirrors deriving from an external MRW-generated type).
+            var systemType = new CSharpType(typeof(FakeMrwBase));
+            var systemBase = new SystemObjectModelProvider(systemType, baseInputModel);
+
+            var generator = MockHelpers.LoadMockGenerator(
+                inputModels: () => [baseInputModel, derivedInputModel],
+                createModelCore: (model) =>
+                {
+                    if (model.Name == "Resource")
+                        return systemBase;
+                    return new ModelProvider(model);
+                },
+                createSerializationsCore: (inputType, typeProvider) =>
+                    inputType is InputModelType modelType && typeProvider is ModelProvider mp
+                        ? [new MrwSerializationTypeDefinition(modelType, mp)]
+                        : []);
+            generator.Object.TypeFactory.RootInputModels.Add(derivedInputModel);
+            generator.Object.TypeFactory.RootOutputModels.Add(derivedInputModel);
+
+            var derived = ScmCodeModelGenerator.Instance.TypeFactory.CreateModel(derivedInputModel) as ModelProvider;
+            Assert.IsNotNull(derived);
+            Assert.IsInstanceOf<SystemObjectModelProvider>(derived!.BaseModelProvider);
+
+            var serializations = derived.SerializationProviders;
+            Assert.AreEqual(1, serializations.Count);
+            return (derived, (MrwSerializationTypeDefinition)serializations[0]);
+        }
+
+        // -------------------------------------------------------------------
+        // When the external/system base itself follows the generated MRW pattern (declares the
+        // protected virtual *Core methods), the derived model must OVERRIDE them rather than
+        // re-introduce them as virtual. Regression test for issue #11042 where deriving from an
+        // external MRW-generated base (e.g. OpenAI's ResponseItem) produced CS0114.
+        // -------------------------------------------------------------------
+
+        [Test]
+        public void PersistableModelCreateCore_IsOverride_WhenSystemBaseDeclaresCoreMethods()
+        {
+            var (_, serialization) = CreateDerivedModelWithMrwSystemBase();
+            var method = serialization.BuildPersistableModelCreateCoreMethod();
+
+            Assert.IsNotNull(method);
+            Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
+                "PersistableModelCreateCore should be 'override' when the system base declares the *Core methods");
+            Assert.IsFalse(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Virtual),
+                "PersistableModelCreateCore should NOT be 'virtual' when the system base declares the *Core methods");
+        }
+
+        [Test]
+        public void JsonModelCreateCore_IsOverride_WhenSystemBaseDeclaresCoreMethods()
+        {
+            var (_, serialization) = CreateDerivedModelWithMrwSystemBase();
+            var method = serialization.BuildJsonModelCreateCoreMethod();
+
+            Assert.IsNotNull(method);
+            Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
+                "JsonModelCreateCore should be 'override' when the system base declares the *Core methods");
+            Assert.IsFalse(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Virtual),
+                "JsonModelCreateCore should NOT be 'virtual' when the system base declares the *Core methods");
+        }
+
+        [Test]
+        public void PersistableModelWriteCore_IsOverride_WhenSystemBaseDeclaresCoreMethods()
+        {
+            var (_, serialization) = CreateDerivedModelWithMrwSystemBase();
+            var method = serialization.BuildPersistableModelWriteCoreMethod();
+
+            Assert.IsNotNull(method);
+            Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
+                "PersistableModelWriteCore should be 'override' when the system base declares the *Core methods");
+            Assert.IsFalse(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Virtual),
+                "PersistableModelWriteCore should NOT be 'virtual' when the system base declares the *Core methods");
+        }
+
+        [Test]
+        public void JsonModelWriteCore_IsOverride_WhenSystemBaseDeclaresCoreMethods()
+        {
+            var (_, serialization) = CreateDerivedModelWithMrwSystemBase();
+            var method = serialization.BuildJsonModelWriteCoreMethod();
+
+            Assert.IsNotNull(method);
+            Assert.IsTrue(method.Signature.Modifiers.HasFlag(MethodSignatureModifiers.Override),
+                "JsonModelWriteCore should be 'override' when the system base declares the *Core methods");
+        }
+
+        /// <summary>
+        /// A stand-in for an external framework type generated by the MRW emitter. It implements
+        /// <see cref="IJsonModel{T}"/> (and therefore <see cref="IPersistableModel{T}"/>) just as a
+        /// generated model does, so the base is recognized as participating in the serialization
+        /// override chain.
+        /// </summary>
+        private class FakeMrwBase : IJsonModel<FakeMrwBase>
+        {
+            void IJsonModel<FakeMrwBase>.Write(Utf8JsonWriter writer, ModelReaderWriterOptions options)
+            {
+            }
+
+            FakeMrwBase IJsonModel<FakeMrwBase>.Create(ref Utf8JsonReader reader, ModelReaderWriterOptions options)
+                => this;
+
+            BinaryData IPersistableModel<FakeMrwBase>.Write(ModelReaderWriterOptions options)
+                => BinaryData.FromString(string.Empty);
+
+            FakeMrwBase IPersistableModel<FakeMrwBase>.Create(BinaryData data, ModelReaderWriterOptions options)
+                => this;
+
+            string IPersistableModel<FakeMrwBase>.GetFormatFromOptions(ModelReaderWriterOptions options)
+                => "J";
+        }
     }
 }