fix(mcp): harden remote SSE fallback

[wangcham]

概述 / Overview

请在此部分填写你实现/解决/优化的内容:
Summary of what you implemented/solved/optimized:

更改前后对比截图 / Screenshots

请在此部分粘贴更改前后对比截图（可以是界面截图、控制台输出、对话截图等）:
Please paste the screenshots of changes before and after here (can be interface screenshots, console output, conversation screenshots, etc.):

修改前 / Before:

修改后 / After:

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?
• 我已签署或将在机器人提示后签署 CLA。 / I have signed, or will sign when prompted by the bot, the CLA.
• 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?