[Bug]: onebot频道定时任务无法发送到群

## CoPaw Version

v1.0.1

## Description

onebot频道里定时任务消息无法发送到群。dispatch里面user_id配置为群ID时，napcat报找不到用户，填群内某个用户id时，无论session_id如何填，都会发送到私聊。
按照napcat的文档，user_id和group_id应该是两个字段

last_dispatch 不保存群信息：只保存 channel、user_id、session_id，没有 is_group、group_id
应该大约是这个原因。

**Related PR(s):** #(optional)

**Security considerations:** [If applicable, e.g. auth, env/config exposure]

## Component(s) Affected

- [ ] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [x] Channels (onebot)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Environment

- **CoPaw version:** v1.0.1
- **OS:** Ubuntu 22.04
- **Install method:** pip
- **Python version (if applicable):** [e.g. 3.10]