fix: report Discord receiver failures and clean up tasks

[wcqqq1214]

Motivation: During local testing, the bot stopped responding to Discord mentions while the process and WebUI remained responsive, prompting this lifecycle investigation; the cause of that specific incident remains unconfirmed.

Discord receiver failures leave the adapter marked running after it stops receiving messages. This change reports unexpected exits as error and releases resources before reload.

Modifications / 改动点

• Supervise the receiver and distinguish unexpected exits from requested shutdown.

• Centralize task/client cleanup and add nine lifecycle regression cases.

• 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

uv run --no-sync pytest tests/test_discord{_lifecycle,_adapter,_command_sync}.py -q
# 22 passed
uv run --no-sync ruff format . tests/test_discord_lifecycle.py
uv run --no-sync ruff check . tests/test_discord_lifecycle.py
# All checks passed

Tests use simulated Discord clients with the real adapter and platform manager.

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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 checked the affected WebUI instructions and screenshots in docs/zh and docs/en against the changed navigation, page structure, and labels, and updated them in this PR (or explained why no documentation update is needed). For renamed, moved, or merged entry points, I included an old entry → new entry mapping in the documentation and changelog.
  / 我已对照变化后的 WebUI 入口、页面结构和术语，核对并在本 PR 中更新 docs/zh 和 docs/en 的相关操作说明与截图（或说明无需更新文档的原因）。入口改名、移动或合并时，已在文档和 changelog 中补充 旧入口 → 新入口 对照。

• 🤓 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

Improve Discord adapter lifecycle handling by surfacing receiver failures and reliably cleaning up resources during shutdown and reload.

Bug Fixes:

• Report unexpected Discord receiver exits as adapter errors so failed receivers no longer remain marked as running.
• Ensure failed Discord adapters can be reloaded cleanly and resume receiving messages.

Enhancements:

• Centralize Discord shutdown cleanup so polling tasks, command registrations, and client connections are consistently released across normal shutdown, cancellation, and receiver failures.

Tests:

• Add lifecycle regression coverage for receiver failures, shutdown and cancellation paths, cleanup failures, and post-reload message delivery.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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