Fix conversation bug in agent session (#4078)

### What problem does this PR solve?

Fix conversation bug in agent session

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "ragflow"
-version = "0.14.0"
+version = "0.15.0"
 description = "[RAGFlow](https://ragflow.io/) is an open-source RAG (Retrieval-Augmented Generation) engine based on deep document understanding. It offers a streamlined RAG workflow for businesses of any scale, combining LLM (Large Language Models) to provide truthful question-answering capabilities, backed by well-founded citations from various complex formatted data."
 authors = ["Your Name <you@example.com>"]
 license = "https://github.com/infiniflow/ragflow/blob/main/LICENSE"
@@ -31,7 +31,7 @@ dashscope = "1.20.11"
 deepl = "1.18.0"
 demjson3 = "3.0.6"
 discord-py = "2.3.2"
-duckduckgo-search = "6.4.1"
+duckduckgo-search = "6.1.9"
 editdistance = "0.8.1"
 elastic-transport = "8.12.0"
 elasticsearch = "8.12.1"