fix: cot gent duplicate messages (langgenius#4470)

Author: Yeuoly

api/core/agent/cot_agent_runner.py

@@ -189,7 +189,7 @@ def increase_usage(final_llm_usage_dict: dict[str, LLMUsage], usage: LLMUsage):
 
             if not scratchpad.action:
                 # failed to extract action, return final answer directly
-                final_answer = scratchpad.agent_response or ''
+                final_answer = ''
             else:
                 if scratchpad.action.action_name.lower() == "final answer":
                     # action is final answer, return final answer directly