[Response Ops] [Alerting] Unflattening summarized alerts (#147890)

Towards #147379

## Summary

When investigating how to [onboard detection alerts onto framework alert
summaries](#147379), there were
some discrepancies in the format of the alert documents returned. This
PR fixes the formatting so it matches and there will be no difference in
`context.alerts` when we migrate detection alerts to the framework.


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios