feat(trainer): add tail_lines support to get_job_logs

[Sumitha007]

What this PR does / why we need it:

This PR adds tail_lines support to TrainerClient.get_job_logs() and propagates it through all Trainer backends.

Changes

• Add tail_lines: int | None = None to TrainerClient.get_job_logs().
• Propagate the new parameter through the RuntimeBackend interface.
• Support tail_lines in the Kubernetes backend by forwarding it to the Kubernetes Pod Logs API.
• Support tail_lines in the Container backend and Docker/Podman adapters.
• Support tail_lines in the LocalProcess backend and LocalJob.logs().
• Preserve existing behavior when tail_lines is not specified.

Which issue(s) this PR fixes:

Fixes #682

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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