fix the issue that codegentype cannot change namespace of resource/resource collection (#5394)

Author: ArcturusZhang

src/AutoRest.CSharp/Mgmt/Generation/MgmtClientBaseWriter.cs

@@ -56,7 +56,7 @@ protected MgmtClientBaseWriter(CodeWriter writer, MgmtTypeProvider provider)
 
         public virtual void Write()
         {
-            using (_writer.Namespace(This.Namespace))
+            using (_writer.Namespace(This.Declaration.Namespace))
             {
                 WriteClassDeclaration();
                 using (_writer.Scope())

src/AutoRest.CSharp/Mgmt/Output/MgmtTypeProvider.cs

@@ -68,9 +68,7 @@ protected MgmtTypeProvider(string resourceName) : base(MgmtContext.Context)
 
         public virtual FormattableString BranchIdVariableName => $"Id";
 
-        public string Namespace => DefaultNamespace;
-
-        public virtual string DiagnosticNamespace => Namespace;
+        public virtual string DiagnosticNamespace => Declaration.Namespace;
 
         public abstract CSharpType? BaseType { get; }
 

test/TestProjects/MgmtCustomizations/src/Customization/PetStoreCollection.cs

@@ -0,0 +1,12 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#nullable disable
+
+using Azure.Core;
+
+namespace MgmtCustomizations.Pets
+{
+    [CodeGenType("PetStoreCollection")]
+    public partial class PetStoreCollection {}
+}

test/TestProjects/MgmtCustomizations/src/Customization/PetStoreResource.cs

@@ -0,0 +1,12 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#nullable disable
+
+using Azure.Core;
+
+namespace MgmtCustomizations.Pets
+{
+    [CodeGenType("PetStoreResource")]
+    public partial class PetStoreResource {}
+}

test/TestProjects/MgmtCustomizations/src/Generated/Extensions/MgmtCustomizationsExtensions.cs

@@ -13,6 +13,7 @@
 using Azure.ResourceManager;
 using Azure.ResourceManager.Resources;
 using MgmtCustomizations.Mocking;
+using MgmtCustomizations.Pets;
 
 namespace MgmtCustomizations
 {

test/TestProjects/MgmtCustomizations/src/Generated/Extensions/MockableMgmtCustomizationsArmClient.cs

@@ -7,6 +7,7 @@
 
 using Azure.Core;
 using Azure.ResourceManager;
+using MgmtCustomizations.Pets;
 
 namespace MgmtCustomizations.Mocking
 {

test/TestProjects/MgmtCustomizations/src/Generated/Extensions/MockableMgmtCustomizationsResourceGroupResource.cs

@@ -11,6 +11,7 @@
 using Azure;
 using Azure.Core;
 using Azure.ResourceManager;
+using MgmtCustomizations.Pets;
 
 namespace MgmtCustomizations.Mocking
 {

test/TestProjects/MgmtCustomizations/src/Generated/LongRunningOperation/PetStoreOperationSource.cs

@@ -11,6 +11,7 @@
 using Azure;
 using Azure.Core;
 using Azure.ResourceManager;
+using MgmtCustomizations.Pets;
 
 namespace MgmtCustomizations
 {

test/TestProjects/MgmtCustomizations/src/Generated/Models/MgmtCustomizationsContext.cs

@@ -9,6 +9,7 @@
 using Azure;
 using Azure.ResourceManager.Models;
 using MgmtCustomizations.Models;
+using MgmtCustomizations.Pets;
 
 namespace MgmtCustomizations
 {