[🔌 Provider]: DingTalk (钉钉) notification provider

[loongmiaow-pixel]

DingTalk (钉钉) is Alibaba's workplace app with 500M+ users, used by most Chinese enterprises.

This is the same webhook-based notification pattern as Slack/Discord/Feishu. API docs: https://open.dingtalk.com/document/robots/custom-robot-access

Ready to implement. /attempt #TODO

[anxovatomica]

PR #6450 includes the DingTalk (钉钉) provider implementation. Supports robot webhook with text/markdown messages, @mentions, and optional signature verification. Ready for review!

/claim #6397