Fix the get mockable client methods to be private (Azure#50978)

* fix the issue

* mockable client should be public

Author: ArcturusZhang

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

@@ -81,18 +81,21 @@ private IReadOnlyList<TypeProvider> BuildExtensions(IReadOnlyList<ResourceClient
                 scopeCandidates[resource.ResourceScope].Add(resource);
             }
 
+            var mockableArmClientResource = new MockableArmClientProvider(typeof(ArmClient), resources);
             var mockableResources = new List<MockableResourceProvider>(scopeCandidates.Count)
             {
                 // add the arm client mockable resource
-                new MockableArmClientProvider(typeof(ArmClient), resources)
+                mockableArmClientResource
             };
+            ManagementClientGenerator.Instance.AddTypeToKeep(mockableArmClientResource.Name);
 
             foreach (var (scope, candidates) in scopeCandidates)
             {
                 if (candidates.Count > 0)
                 {
                     var mockableExtension = new MockableResourceProvider(_scopeToTypes[scope], candidates);
                     mockableResources.Add(mockableExtension);
+                    ManagementClientGenerator.Instance.AddTypeToKeep(mockableExtension.Name);
                 }
             }
 

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

@@ -64,7 +64,7 @@ private MethodProvider BuildGetCachedClientMethod(MockableResourceProvider mocka
             var methodSignature = new MethodSignature(
                 $"Get{mockableResource.Name}",
                 null,
-                MethodSignatureModifiers.Public | MethodSignatureModifiers.Static,
+                MethodSignatureModifiers.Private | MethodSignatureModifiers.Static,
                 mockableResource.Type,
                 null,
                 [parameter]);