Log timings in vf-eval

[mikasenghaas]

Description

This PR adds logging for rollout times invf-eval. Logs min/mean/max recorded timings in human-readable format

uv run vf-eval math-python -n1 -r1 -v

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Adds concise rollout timing stats to evaluation output.

• In eval_utils.print_results, aggregates generation_ms, scoring_ms, and total_ms, converts to seconds, and prints min/mean/max via print_time
• Introduces print_time in logging_utils to format seconds into human-readable strings and updates import usage

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