[Pipelines] Reflect the status of downstream pipeline runs in the parent run

For chained pipeline runs like autorest preview, the pipeline run will queue other pipelines or create a PR that causes pr validation runs to be queues, then complete successfully even if the child pipeline runs fail. It would be helpful to be able to have the failed child runs be reflected in the parent pipeline run.

As a concrete example, when [`autorest python - preview`](https://dev.azure.com/azure-sdk/internal/_build?definitionId=6420) runs, it builds and internally publishes a preview version of the emitter and creates [a pull request in azure-sdk-for-python](https://github.com/Azure/azure-sdk-for-python/pull/33774) that consumes the preview emitter. If the pipeline successfully creates the PR, then it completes with a green success status. Asynchronously, the PR triggers child builds that may fail. This will be reflected as a bad/incomplete checksuite in the PR, but the originating pipeline run won't reflect this state.

The issue names no repository files, tests, or entry points. Start by tracing how chained runs and pull-request-triggered child builds are created, then define completion as a parent run reporting failure when an asynchronously triggered child run fails.

Written by the indexing model from the issue text.