Refactor StackReferenceMock filtering in Mocks.cs

Modified the filtering of StackReferenceMock instances in Mocks.cs in order to simplify and improve code readability. Refactored the two separate Where clauses into OfType<StackReferenceMock>() followed by a Where clause, which provides the same functionality but in a more clear and concise manner. Also removed redundant newlines at the end of the file.

PulumiTestHelper/Internals/Mocks.cs

@@ -68,9 +68,10 @@ private Dictionary<string, object> GetResourceMocks(MockResourceArgs args)
     private void AddStackReferenceMocks(MockResourceArgs args, ImmutableDictionary<string, object>.Builder outputs)
     {
         var mocks = resourceMocks
-            .Where(x => x is StackReferenceMock)
-            .Where(x => ((StackReferenceMock) x).StackReferenceName == args.Name)
-            .Select(x => x.Mocks).DeepMergeMany();
+            .OfType<StackReferenceMock>()
+            .Where(x => x.StackReferenceName == args.Name)
+            .Select(x => x.Mocks)
+            .DeepMergeMany();
 
         outputs.Add("secretOutputNames", new List<string>());
         outputs.Add("outputs", mocks);
@@ -86,6 +87,4 @@ private Dictionary<string, object> GetCallMocks(MockCallArgs args)
 
         return mocks.DeepMergeInto(mocksFromFunc);
     }
-
-
 }