Add entry to changelog

Author: alextwoods

.changes/2.40.13.json

@@ -2,6 +2,12 @@
     "version": "2.40.13",
     "date": "2025-12-19",
     "entries": [
+        {
+            "type": "bugfix",
+            "category": "AWS SDK for Java v2",
+            "contributor": "",
+            "description": "Downgrade Jackson and Jackson JR to 2.18.5 to address parsing errors in large strings."
+        },
         {
             "type": "bugfix",
             "category": "AWS SDK for Java v2",