Restore Failure from External Checkpoint during Upgrade

## overview
Jobs that enable DELETE_ON_CANCELLATION for externalized checkpoints will fail during upgrades if the operator attempts to find an externalized checkpoint. The checkpoint directory exists but the _metadata file has been deleted and the job fails to start as its unable to find the _metadata file.

When looking for externalized checkpoints, we should ensure that there is a _metadata file before starting the job with it

Start by tracing how the operator searches for externalized checkpoints during an upgrade, focusing on the checkpoint directory and its _metadata file. Confirm the behavior when _metadata is absent; done means an invalid external checkpoint is not selected and the job can start using a valid checkpoint or the normal fallback.

Written by the indexing model from the issue text.