Notify observer to complete async stream (#5008)

src/StrawberryShake/Client/src/Core/OperationExecutor.Observable.cs

@@ -129,6 +129,10 @@ public IDisposable Subscribe(IObserver<IOperationResult<TResult>> observer)
             }
             finally
             {
+                // call observer's OnCompleted method to notify observer
+                // there is no further data is available.
+                observer.OnCompleted();
+
                 // after all the transport logic is finished we will dispose
                 // the request session.
                 session.RequestSession.Dispose();