limit the system context length of conversation messages. (#962)

### What problem does this PR solve?

#951 

### Type of change

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

api/db/services/dialog_service.py

 
     kwargs["knowledge"] = "\n".join(knowledges)
     gen_conf = dialog.llm_setting
-    msg = [{"role": m["role"], "content": m["content"]}
-           for m in messages if m["role"] != "system"]
+    
+    msg = [{"role": "system", "content": prompt_config["system"].format(**kwargs)}]
+    msg.extend([{"role": m["role"], "content": m["content"]}
+           for m in messages if m["role"] != "system"])
     used_token_count, msg = message_fit_in(msg, int(max_tokens * 0.97))
+    assert len(msg) >= 2, f"message_fit_in has bug: {msg}"
+
     if "max_tokens" in gen_conf:
         gen_conf["max_tokens"] = min(
             gen_conf["max_tokens"],
 
     if stream:
         answer = ""
-        for ans in chat_mdl.chat_streamly(prompt_config["system"].format(**kwargs), msg, gen_conf):
+        for ans in chat_mdl.chat_streamly(msg[0]["content"], msg[1:], gen_conf):
             answer = ans
             yield {"answer": answer, "reference": {}}
         yield decorate_answer(answer)
     else:
         answer = chat_mdl.chat(
-            prompt_config["system"].format(
-                **kwargs), msg, gen_conf)
+            msg[0]["content"], msg[1:], gen_conf)
         chat_logger.info("User: {}|Assistant: {}".format(
             msg[-1]["content"], answer))
         yield decorate_answer(answer)