Minor codegen improvements in Microsoft.Toolkit.Collections APIs

Microsoft.Toolkit/Collections/ObservableGroupedCollectionExtensions.cs

@@ -444,7 +444,6 @@ private static void RemoveItemAtWithLinq<TKey, TValue>(
         /// <summary>
         /// Throws a new <see cref="InvalidOperationException"/> when a key is not found.
         /// </summary>
-        [MethodImpl(MethodImplOptions.NoInlining)]
         private static void ThrowArgumentExceptionForKeyNotFound()
         {
             throw new InvalidOperationException("The requested key was not present in the collection");

Microsoft.Toolkit/Collections/ReadOnlyObservableGroupedCollection.cs

@@ -52,8 +52,16 @@ private void OnSourceCollectionChanged(object sender, NotifyCollectionChangedEve
             // reporting the changes one by one. We consider only this case for now.
             if (e.OldItems?.Count > 1 || e.NewItems?.Count > 1)
             {
-                Debug.Fail("OldItems and NewItems should contain at most 1 item");
-                throw new NotSupportedException();
+                static void ThrowNotSupportedException()
+                {
+                    throw new NotSupportedException(
+                        "ReadOnlyObservableGroupedCollection<TKey, TValue> doesn't support operations on multiple items at once.\n" +
+                        "If this exception was thrown, it likely means support for batched item updates has been added to the " +
+                        "underlying ObservableCollection<T> type, and this implementation doesn't support that feature yet.\n" +
+                        "Please consider opening an issue in https://aka.ms/windowstoolkit to report this.");
+                }
+
+                ThrowNotSupportedException();
             }
 
             var newItem = e.NewItems?.Cast<ObservableGroup<TKey, TValue>>()?.FirstOrDefault();