fix: fix post process answer, add self parameter

app.py

@@ -19,7 +19,7 @@ class BasicAgent:
     def __init__(self):
         print("BasicAgent initialized.")
 
-    def post_process_answer(answer: str):
+    def post_process_answer(self, answer: str):
         if "FINAL ANSWER:" not in answer:
             raise ValueError("The answer does not contain 'FINAL ANSWER:' keyword")
         key_answer_start_idx = answer.find("FINAL ANSWER:") + len("FINAL ANSWER:")