fix(spark): normalize non-streaming pod log lines

[lntutor]

Summary

• normalize non-streaming Kubernetes pod logs with splitlines()
• avoid synthetic blank entries for empty or newline-terminated responses
• handle CRLF responses while preserving genuine interior blank lines

Fixes #656

Validation

• uv run pytest -q kubeflow/spark/backends/kubernetes/utils_test.py
• make verify
• git diff --check

[google-oss-prow]

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