chore: refurish python code by applying Pylint linter rules (#8322)

api/models/model.py

@@ -134,7 +134,7 @@ class App(db.Model):
             return False
         if self.app_model_config.agent_mode_dict.get("enabled", False) and self.app_model_config.agent_mode_dict.get(
             "strategy", ""
-        ) in ["function_call", "react"]:
+        ) in {"function_call", "react"}:
             self.mode = AppMode.AGENT_CHAT.value
             db.session.commit()
             return True
@@ -1501,6 +1501,6 @@ class TraceAppConfig(db.Model):
             "tracing_provider": self.tracing_provider,
             "tracing_config": self.tracing_config_dict,
             "is_active": self.is_active,
-            "created_at": self.created_at.__str__() if self.created_at else None,
-            "updated_at": self.updated_at.__str__() if self.updated_at else None,
+            "created_at": str(self.created_at) if self.created_at else None,
+            "updated_at": str(self.updated_at) if self.updated_at else None,
         }