[Azure Mgmt Generator] Add ProviderConstantsProvider to generate ProviderConstants.cs files (Azure#51116)

Author: Copilot

eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/ManagementOutputLibrary.cs

@@ -29,6 +29,9 @@ public class ManagementOutputLibrary : AzureOutputLibrary
         private PageableWrapperProvider? _asyncPageableWrapper;
         internal PageableWrapperProvider AsyncPageableWrapper => _asyncPageableWrapper ??= new PageableWrapperProvider(true);
 
+        private ProviderConstantsProvider? _providerConstants;
+        internal ProviderConstantsProvider ProviderConstants => _providerConstants ??= new ProviderConstantsProvider();
+
         private IReadOnlyList<ResourceClientProvider>? _resourceClients;
         internal IReadOnlyList<ResourceClientProvider> ResourceClients => _resourceClients ??= BuildResources();
 
@@ -153,6 +156,7 @@ protected override TypeProvider[] BuildTypeProviders()
                 .. base.BuildTypeProviders().Where(t => t is not SystemObjectModelProvider),
                 ArmOperation,
                 ArmOperationOfT,
+                ProviderConstants,
                 .. resources,
                 .. collections,
                 .. extensions,

eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Providers/MockableResourceProvider.cs

@@ -67,10 +67,9 @@ private static Dictionary<InputClient, RestClientInfo> BuildRestClientInfos(IRea
                     MethodSignatureModifiers.Private,
                     typeof(ClientDiagnostics),
                     ResourceHelpers.GetClientDiagnosticsPropertyName(restClientProvider.Name),
-                    // TODO -- here we either need to use the corresponding ResourceType.Namespace or ProviderConstants.DefaultProviderNamespace, pending on https://github.com/Azure/azure-sdk-for-net/issues/50593
                     new ExpressionPropertyBody(
                         clientDiagnosticsField.Assign(
-                            New.Instance(typeof(ClientDiagnostics), Literal(enclosingType.Type.Namespace), Literal("TODO"), thisResource.Diagnostics()),
+                            New.Instance(typeof(ClientDiagnostics), Literal(enclosingType.Type.Namespace), ProviderConstantsProvider.DefaultProviderNamespace, thisResource.Diagnostics()),
                             nullCoalesce: true)),
                     enclosingType);
 

eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Providers/ProviderConstantsProvider.cs

@@ -0,0 +1,38 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Azure.Core.Pipeline;
+using Microsoft.TypeSpec.Generator.Expressions;
+using Microsoft.TypeSpec.Generator.Primitives;
+using Microsoft.TypeSpec.Generator.Providers;
+using System.IO;
+using static Microsoft.TypeSpec.Generator.Snippets.Snippet;
+
+namespace Azure.Generator.Management.Providers
+{
+    internal class ProviderConstantsProvider : TypeProvider
+    {
+        private const string DefaultProviderNamespaceName = "DefaultProviderNamespace";
+
+        public static ValueExpression DefaultProviderNamespace =>  Static(ManagementClientGenerator.Instance.OutputLibrary.ProviderConstants.Type).Property(DefaultProviderNamespaceName);
+
+        protected override string BuildName() => "ProviderConstants";
+
+        protected override string BuildRelativeFilePath() => Path.Combine("src", "Generated", $"{Name}.cs");
+
+        protected override TypeSignatureModifiers BuildDeclarationModifiers() => TypeSignatureModifiers.Internal | TypeSignatureModifiers.Static;
+
+        protected override PropertyProvider[] BuildProperties()
+        {
+            var defaultProviderNamespaceProperty = new PropertyProvider(
+                null,
+                MethodSignatureModifiers.Public | MethodSignatureModifiers.Static,
+                typeof(string),
+                DefaultProviderNamespaceName,
+                new ExpressionPropertyBody(Static<ClientDiagnostics>().Invoke(nameof(ClientDiagnostics.GetResourceProviderNamespace), [TypeOf(Type).Property(nameof(System.Type.Assembly))])),
+                this);
+
+            return [defaultProviderNamespaceProperty];
+        }
+    }
+}