Add missing prompt in template

The "final_answer" key was missing in the template prompt. I added the key in order to fix the current error obtained in the template

prompts.yaml

@@ -319,3 +319,10 @@
   "report": |-
     Here is the final answer from your managed agent '{{name}}':
     {{final_answer}}
+"final_answer":
+  "pre_messages": |-
+    Here is the final answer from your managed agent '{{name}}':
+    {{final_answer}}
+  "post_messages": |-
+    Here is the final answer from your managed agent '{{name}}':
+    {{final_answer}}