Add api for list agents and agent seesions (#3835)

### What problem does this PR solve?

Add api for list agents and agent seesions

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

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

api/apps/canvas_app.py

@@ -65,7 +65,7 @@ def save():
     req["dsl"] = json.loads(req["dsl"])
     if "id" not in req:
         if UserCanvasService.query(user_id=current_user.id, title=req["title"].strip()):
-            return server_error_response(ValueError("Duplicated title."))
+            return get_data_error_result(f"{req['title'].strip()} already exists.")
         req["id"] = get_uuid()
         if not UserCanvasService.save(**req):
             return get_data_error_result(message="Fail to save canvas.")