Remove unnecessary .ToList() snapshot from RewriteRefPointers

Address review feedback: the  assignment completes before the
foreach enumerator is created, and recursive calls only mutate
descendant objects, so snapshotting via .ToList() is not needed.
This makes the JsonObject and JsonArray iteration consistent.

Author: weinong

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -581,8 +581,9 @@ private static void RewriteRefPointers(JsonNode? node)
                 }
             }
 
-            // ToList() creates a snapshot because the $ref assignment above may invalidate the enumerator.
-            foreach (var property in obj.ToList())
+            // Safe to iterate without snapshot: the $ref assignment above completes before
+            // this enumerator is created, and recursive calls only mutate descendant objects.
+            foreach (var property in obj)
             {
                 RewriteRefPointers(property.Value);
             }