[Fusion] Refactored merging to ensure correct type references (#7995)

Author: glen-84

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/GroupingExtensions.cs

@@ -0,0 +1,13 @@
+namespace HotChocolate.Fusion.Extensions;
+
+internal static class GroupingExtensions
+{
+    public static void Deconstruct<TKey, TElement>(
+        this IGrouping<TKey, TElement> grouping,
+        out TKey key,
+        out IEnumerable<TElement> values)
+    {
+        key = grouping.Key;
+        values = grouping;
+    }
+}