Fix bugs in api (#3624)

### What problem does this PR solve?

#3488

### Type of change

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

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

api/apps/sdk/chat.py

         req['prompt_config'] = {}
     for key in key_list_2:
         temp = req['prompt_config'].get(key)
-        if not temp:
+        if (not temp and key == 'system') or (key not in req["prompt_config"]):
             req['prompt_config'][key] = default_prompt[key]
     for p in req['prompt_config']["parameters"]:
         if p["optional"]:

api/apps/sdk/session.py

                                 "document_id": chunk["doc_id"],
                                 "document_name": chunk["docnm_kwd"],
                                 "dataset_id": chunk["kb_id"],
-                                "image_id": chunk["img_id"],
+                                "image_id": chunk["image_id"],
                                 "similarity": chunk["similarity"],
                                 "vector_similarity": chunk["vector_similarity"],
                                 "term_similarity": chunk["term_similarity"],

sdk/python/ragflow_sdk/modules/session.py

                 self.__session_type = "agent"
         super().__init__(rag, res_dict)
 
-    def ask(self, question):
+    def ask(self, question,stream=True):
         if self.__session_type == "agent":
-            res=self._ask_agent(question)
+            res=self._ask_agent(question,stream)
         elif self.__session_type == "chat":
-            res=self._ask_chat(question)
+            res=self._ask_chat(question,stream)
         for line in res.iter_lines():
             line = line.decode("utf-8")
             if line.startswith("{"):
                     yield message
 
 
-    def _ask_chat(self, question: str, stream: bool = False):
+    def _ask_chat(self, question: str, stream: bool):
         res = self.post(f"/chats/{self.chat_id}/completions",
                         {"question": question, "stream": True,"session_id":self.id}, stream=stream)
         return res
-    def _ask_agent(self,question:str,stream:bool=False):
+    def _ask_agent(self,question:str,stream:bool):
         res = self.post(f"/agents/{self.agent_id}/completions",
                         {"question": question, "stream": True,"session_id":self.id}, stream=stream)
         return res