fix: remove stale platform instance on reload

[Clhikari]

修复了平台热重载时，旧适配器实例可能未被正确移除的问题。

在现有实现里，PlatformManager.terminate_platform() 会通过 client_self_id 从 platform_insts 中查找并移除实例。这个字段在 Discord 适配器运行过程中可能发生变化，导致热重载时旧实例未能被正确摘除。后续主动发送消息时，就有机会继续命中已经关闭的旧实例，并触发 Session is closed 一类错误。

Modifications / 改动点

• 修改了 astrbot/core/platform/manager.py

• 在 terminate_platform() 中，改为直接移除 _inst_map 中记录的实例对象 info["inst"]

• 保留原有终止和任务清理流程，未扩展到通用发送链路或 Discord 发送实现

• 新增回归测试 tests/unit/test_platform_manager.py

• 测试覆盖了 client_self_id 变化后，旧实例仍能被正确移除的场景

• 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

本地已执行以下验证步骤：

uv run ruff format astrbot/core/platform/manager.py tests/unit/test_platform_manager.py
uv run ruff check astrbot/core/platform/manager.py tests/unit/test_platform_manager.py
uv run python -m pytest tests/unit/test_platform_manager.py -q

测试结果：

1 passed

本次回归测试验证了以下内容：

• 热重载终止平台时，移除逻辑不再依赖 client_self_id 匹配
• 即使实例运行期间 client_self_id 发生变化，terminate_platform() 仍然会移除正确的旧实例
• 终止流程仍会继续调用后续的实例清理逻辑

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Fixes #7187

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。

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[gemini-code-assist]

Code Review

This pull request simplifies the platform termination process in PlatformManager by directly removing the adapter instance from the tracking list, ensuring the operation remains safe even if the client ID changes. A new unit test has been added to verify this fix. The reviewer suggested improving the robustness of the error handling by catching a specific ValueError instead of a broad Exception when removing the instance.