fix: RubricGroup scoring_ms should be the time to score all rubrics

[pjo256]

Description

Measure total scoring time once at the RubricGroup level. scoring_ms was previously the last rubric's scoring_ms in a RubricGroup.

Fixes #952.

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

Low Risk
Low risk: isolated change to timing bookkeeping in RubricGroup, plus tests validating scoring_ms/total_ms consistency.

Overview
Fixes RubricGroup timing so scoring_ms reflects the total time spent scoring across all rubrics (rather than whatever the last rubric wrote). score_rollout and score_group now measure wall-clock time once at the group level, restore each state’s original timing between rubric evaluations, and then update scoring_ms and increment total_ms after aggregation.

Adds async tests asserting generation_ms + scoring_ms == total_ms for both score_rollout and score_group.

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