docs: 补充隔离会话对 unified_msg_origin 格式影响的说明

[lingyun14beta]

当用户开启隔离会话时，群消息的 unified_msg_origin 格式会发生变化，但文档中未说明此行为，导致插件开发者在以群为单位存储数据时出现数据不一致的问题。

Modifications / 改动点

• 在 docs/zh/dev/star/guides/listen-message-event.md 的 AstrBotMessage 说明后添加 WARNING，说明隔离会话对 session_id 和 unified_msg_origin 格式的影响，并建议开发者使用 event.get_group_id() 获取纯群ID。

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Documentation:

• Add a warning explaining how enabling isolated sessions changes group message session_id and unified_msg_origin formats, and recommend using event.get_group_id() for group-based data storage and queries.

[sourcery-ai]

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[gemini-code-assist]

Code Review

This pull request updates the documentation for message event handling to include a warning regarding 'Isolated Sessions'. It explains how session IDs and message origins change when this feature is enabled and advises developers to use the event.get_group_id() method for group-specific data. A review comment suggests extending this warning to explicitly advise against using session_id directly as a group identifier to prevent logic errors.

[lingyun14beta]

session_id 不会因隔离会话而改变的……吧?

[lingyun14beta]

是我考虑不周了