Chat notification when DCAR main build is broken

We currently don't have an automated way of being alerted when DCAR's main build is broken.

For example this happened on this main commit:
https://github.com/guardian/dotcom-rendering/commit/ae0a2ed034ac832a8471951c32b1fc934462fe27

It would be useful to receive a chat notification on the public WebX channel when this happens.

We need to investigate the best method to do this, it could be in the existing build check workflow or a new check.

https://github.com/guardian/dotcom-rendering/actions/runs/18372299100/job/52338144415

https://github.com/guardian/dotcom-rendering/blob/2482041ce59184f2e4cb9a4d9736412d32997ede/.github/workflows/build-check.yml

從 .github/workflows/build-check.yml 以及針對所引用 main commit 的相關失敗 GitHub Actions 執行開始。調查現有的 build check 如何回報失敗，並決定合適的通知方式；完成的標準是，損壞的 DCAR main build 會在公開的 WebX 頻道中產生聊天通知。

由索引模型根據 Issue 內容生成。