fix(trainer): preserve local job runtime metadata

[lntutor]

Summary:

• Return each local job's stored runtime from unfiltered listings.
• Add regression coverage for list_jobs without a runtime filter.

Fixes #653

Validation:

• make test-python
• make verify
• uv run pytest kubeflow/trainer/backends/localprocess/backend_test.py -q

[google-oss-prow]

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[kingabdulsalamsas111-lgtm]

Thanks for picking this up! I reported the issue in #653, and it's great to see both the fix and a regression test included. Looks good to me.