fix: resolve Discord/Misskey hot reload issue by fixing client_self_id misuse

[Soulter]

fixes: #7187
closes: #7188

Modifications / 改动点

• 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

Align bot self-identifier handling across Misskey and Discord adapters and clean up unused client identifiers in other platform adapters to resolve hot-reload and self-id related issues.

Bug Fixes:

• Correct Misskey adapter self-id usage to reliably detect bot messages, mentions, and visibility for events and message conversion.
• Initialize and propagate the Discord bot self-id from received events so outgoing and converted messages use the correct identifier.
• Prevent errors in the Telegram adapter when applying captions on updates without a message payload.

Enhancements:

• Rename client self-id fields to a clearer bot_self_id in Misskey and Discord adapters and update related utilities for consistency.
• Remove unused placeholder client_self_id fields from Telegram, WeCom, and Weixin Official Account adapters.

[sourcery-ai]

Hey - I've left some high level feedback:

• The self ID handling across adapters is still a bit ad hoc; consider centralizing bot/self ID resolution in the common base class (or a shared helper) so that Discord/Misskey/Telegram/WeCom/Weixin don’t each manage their own bot_self_id lifecycle differently and drift again.
• In the Discord adapter, bot_self_id is first set from the first received message inside on_received, which means send_by_session could run while bot_self_id is still None; you may want to initialize it from self.client.user.id once the client is ready instead of lazily on first message.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The self ID handling across adapters is still a bit ad hoc; consider centralizing bot/self ID resolution in the common base class (or a shared helper) so that Discord/Misskey/Telegram/WeCom/Weixin don’t each manage their own `bot_self_id` lifecycle differently and drift again.
- In the Discord adapter, `bot_self_id` is first set from the first received message inside `on_received`, which means `send_by_session` could run while `bot_self_id` is still `None`; you may want to initialize it from `self.client.user.id` once the client is ready instead of lazily on first message.

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[gemini-code-assist]

Code Review

This pull request renames client_self_id to bot_self_id across several platform adapters, including Discord and Misskey, and removes unused ID initializations in the Telegram, WeCom, and WeChat adapters. Feedback identifies a redundant null check added to the Telegram adapter and suggests improvements for the Discord adapter to ensure the bot's identity is retrieved reliably from the client user object and correctly typed as a string.