Fix bugs in agent api and update api document (#3996)

### What problem does this PR solve?

Fix bugs in agent api and update api document

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

---------

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

api/db/services/conversation_service.py

@@ -86,8 +86,9 @@ def completion(tenant_id, chat_id, question, name="New session", session_id=None
     assert dia, "You do not own the chat."
 
     if not session_id:
+        session_id = get_uuid()
         conv = {
-            "id": get_uuid(),
+            "id":session_id ,
             "dialog_id": chat_id,
             "name": name,
             "message": [{"role": "assistant", "content": dia[0].prompt_config.get("prologue")}]