feat(providers): add WeCom (企业微信) notification provider

[ayoubil]

Summary

Adds WeCom (企业微信) notification provider for sending alerts to WeCom group chats via webhook.

Closes #6396

What is included

• keep/providers/wecom_provider/wecom_provider.py — WeCom webhook provider
• docs/providers/documentation/wecom-provider.mdx — documentation
• mint.json navigation entry

Provider details

• Type: Notification (outbound)
• Auth: Webhook URL from WeCom group robot settings
• Supports: Text and Markdown message types
• API: https://developer.work.weixin.qq.com/document/path/91770

Follows the same webhook pattern as the existing Discord and Slack providers.

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