feat: default to custom job display name if experiment name looks like a custom job ID

[yfang1]

The training backend doesn't store custom job display names, so the uploader would need to fetch the job display name on its own.

This allows experiments to be more recognizable in the Cloud Console experiment list UI.

b/203591169

[nicain]

@yfang1: I am assigning as owner of this PR; feel free to ping reviewers as needed to make sure the review process progresses in a timely fashion, or provide guidance on a who might better own the process of getting the PR reviewed, passing continuous testing, and merged. Reach out if you have questions.

[sasha-gitg]

LGTM. Does this have a unit test that can be updated?

This mock can be copied/reused: https://github.com/googleapis/python-aiplatform/blob/main/tests/unit/aiplatform/test_custom_job.py#L147

[yfang1]

LGTM. Does this have a unit test that can be updated?

This mock can be copied/reused: https://github.com/googleapis/python-aiplatform/blob/main/tests/unit/aiplatform/test_custom_job.py#L147

Yes, I added new unit tests for the change.