feat: add Qiniu provider support

[JackChiang233]

概述 / Overview

• Added support for Qiniu AI (七牛云) as a new model provider.
• 添加七牛云(Qiniu)作为新的模型厂商

更改前后对比截图 / Screenshots

修改后 / After:

改变的文件 What changed

• src/langbot/pkg/provider/modelmgr/requesters/qiniuchatcmpl.yaml — Declares the metadata and registration information of the Qiniu Cloud Chat Completions requester (display name, icon, configurables, execution entry class).

• src/langbot/pkg/provider/modelmgr/requesters/qiniuchatcmpl.py — Inherits from OpenAI to be compatible with Chat Completions, pointing to the Qiniu gateway by default; uses a built-in placeholder list as a fallback when model enumeration fails or is empty.

• Add relevant vendor information to readme.md

---

• src/langbot/pkg/provider/modelmgr/requesters/qiniuchatcmpl.yaml — 声明七牛云 Chat Completions 请求器（展示名、图标、可配项、执行入口类）的元数据与注册信息。

• src/langbot/pkg/provider/modelmgr/requesters/qiniuchatcmpl.py — 继承 OpenAI 兼容 Chat Completions，默认指向七牛网关；模型列举失败或为空时用内置占位列表兜底。

• 相关readme.md的厂商添加。

测试 Testing

uv run pytest --cov=langbot --cov-report=term-missing		# all passed

检查清单 / Checklist

PR 作者完成 / For PR author

请在方括号间写x以打勾 / Please tick the box with x

• 阅读仓库贡献指引了吗？ / Have you read the contribution guide?
• 与项目所有者沟通过了吗？ / Have you communicated with the project maintainer?
• 我确定已自行测试所作的更改，确保功能符合预期。 / I have tested the changes and ensured they work as expected.

项目维护者完成 / For project maintainer

• 相关 issues 链接了吗？ / Have you linked the related issues?
• 配置项写好了吗？迁移写好了吗？生效了吗？ / Have you written the configuration items? Have you written the migration? Has it taken effect?
• 依赖加到 pyproject.toml 和 core/bootutils/deps.py 了吗 / Have you added the dependencies to pyproject.toml and core/bootutils/deps.py?
• 文档编写了吗？ / Have you written the documentation?

[RockChinQ]

不用写migration的，可以直接删掉这个迁移