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>

sdk/python/ragflow_sdk/modules/agent.py

@@ -1,7 +1,9 @@
 from .base import Base
-from .session import Session
+from .session import Session,Message
 import requests
 from typing import List
+import json
+
 
 class Agent(Base):
     def __init__(self,rag,res_dict):
@@ -73,3 +75,30 @@ class Agent(Base):
                 result_list.append(temp_agent)
             return result_list
         raise Exception(res.get("message"))
+
+    @staticmethod
+    def ask(agent_id,rag,stream=True,**kwargs):
+        url = f"{rag.api_url}/agents/{agent_id}/completions"
+        headers = {"Authorization": f"Bearer {rag.user_key}"}
+        res = requests.post(url=url, headers=headers, json=kwargs,stream=stream)
+        for line in res.iter_lines():
+            line = line.decode("utf-8")
+            if line.startswith("{"):
+                json_data = json.loads(line)
+                raise Exception(json_data["message"])
+            if line.startswith("data:"):
+                json_data = json.loads(line[5:])
+                if json_data["data"] is not True:
+                    if json_data["data"].get("running_status"):
+                        continue
+                    answer = json_data["data"]["answer"]
+                    reference = json_data["data"]["reference"]
+                    temp_dict = {
+                        "content": answer,
+                        "role": "assistant"
+                    }
+                    if "chunks" in reference:
+                        chunks = reference["chunks"]
+                        temp_dict["reference"] = chunks
+                    message = Message(rag, temp_dict)
+                    yield message

sdk/python/ragflow_sdk/modules/session.py

@@ -29,7 +29,7 @@ class Session(Base):
                 raise Exception(json_data["message"])
             if line.startswith("data:"):
                 json_data = json.loads(line[5:])
-                if not json_data["data"]:
+                if json_data["data"] is not True:
                     answer = json_data["data"]["answer"]
                     reference = json_data["data"]["reference"]
                     temp_dict = {