feat(providers): add WeCom notification provider

[LZG3530606141]

Summary

Implements WeCom (企业微信) notification provider for Keep, as requested in #6396.

What

• New WecomProvider that sends alert notifications to WeCom group chats via Webhook Bot
• Follows the standard Keep notification provider pattern (similar to Slack/Discord/Feishu)

Features

• Text messages: Simple text notifications with @mention support (by user ID or phone number)
• Markdown messages (default): Markdown formatted notifications
• News messages: Article-style messages with title, description, and URL
• @mention support: mentioned_list (WeCom user IDs) and mentioned_mobile_list (phone numbers)

Implementation Details

• Auth config: webhook_url (required)
• Provider category: Collaboration
• Provider tags: messaging
• Auto-discovered via Keep importlib provider factory

Testing

Example workflow:

- provider:
    type: wecom
    config:
      authentication:
        webhook_url: HOOK_URL_HERE
  notify:
    message: "### Keep Alert\n**High CPU usage detected**"
    title: "Keep Alert"

References

• WeCom Webhook Bot API: https://developer.work.weixin.qq.com/document/path/91770
• Issue: [🔌 Provider]: WeCom (企业微信) notification provider #6396

Closes #6396

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