bug(trainer): list_jobs and get_job fail after a referenced runtime is deleted

### What happened?

In the Kubernetes backend, `list_jobs()` and `get_job()` look up each TrainJob's runtime live with `get_runtime()`. If that runtime has been deleted, the lookup raises and `list_jobs()` fails with "Failed to list TrainJobs", so one such job breaks the whole list.

Trainer v2.3.0 removed the runtime finalizers (kubeflow/trainer#3716) and snapshots the runtime per job (KEP-2599), so deleting a runtime while its jobs still exist is now expected.

Found in the code (`kubeflow/trainer/backends/kubernetes/backend.py`, `__get_trainjob_from_cr`), not run on a cluster.

### What did you expect to happen?

`list_jobs()` still returns the other jobs, and `get_job()` works for a job whose runtime is gone.

Should these jobs be skipped, returned without a runtime, or read from the runtime snapshot? I'll open a PR once there's a direction.

### Environment

Kubernetes version: n/a, found in code
Kubeflow Trainer version: v2.3.0
Kubeflow Python SDK version: 0.5.0 and main (3febcd7)

Start in kubeflow/trainer/backends/kubernetes/backend.py, especially __get_trainjob_from_cr and the list_jobs() and get_job() entry points. Read how get_runtime() is used and review the referenced runtime snapshot behavior in KEP-2599. Confirm with maintainers whether missing runtimes should be skipped, omitted, or reconstructed from snapshots; done means the agreed behavior works without one deleted runtime breaking other job lookups.

Written by the indexing model from the issue text.