add assistant to canvas chat history (#3201)

### What problem does this PR solve?

#3185

### Type of change

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

api/apps/canvas_app.py

@@ -133,6 +133,7 @@ def run():
                     yield "data:" + json.dumps({"retcode": 0, "retmsg": "", "data": ans}, ensure_ascii=False) + "\n\n"
 
                 canvas.messages.append({"role": "assistant", "content": final_ans["content"], "id": message_id})
+                canvas.history.append(("assistant", final_ans["content"]))
                 if final_ans.get("reference"):
                     canvas.reference.append(final_ans["reference"])
                 cvs.dsl = json.loads(str(canvas))