Return build failure_reason in pipeline's webhooks

What does this MR do and why?

Adds the failure_reason field to the build object that's returned in the pipeline webhook, similarly to what we did in !74888 (merged)

The field is always present, but null if the job hasn't failed.

I updated the docs with this field (and a couple other missing fields) and added a test.

How to set up and validate locally

1. Create a pipeline webhook and point it to something that logs the requests received, like https://pipedream.com/
2. Trigger a CI run
3. Check the new failure_reason field on items inside the builds array.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.