2.5. mt-bench changes

[ErlisLushtaku]

Summary

This PR pulls the MT-Bench-specific follow-up work out of the prompt preset PR and keeps it as a focused stacked cleanup.

It now does two things:

• refactors the non-delegated MT-Bench judging path to reuse shared pairwise helpers instead of duplicating batching, prompt grouping, answer swapping, and item construction
• adds MT-Bench-specific reproducibility/orchestration follow-ups that were originally mixed into PR 52, without bringing over the deferred truncation tracking or cache-key changes

Concretely, this PR:

• extracts shared MT-Bench pairwise judging helpers into judgearena/mt_bench/pairwise_judging.py
• keeps FastChat-specific verdict parsing and preset-specific score parsing separate, while reusing the same lower-level MT-Bench mechanics
• replaces loose shared item dicts with a typed MTBenchJudgeItem
• makes the two swap_mode="both" semantics explicit in code:
  • FastChat: conservative agreement
  • preset judging: append inverted swapped scores
• factors shared MT-Bench result finalization in mt_bench_utils.py
• aligns pre-generated or cached MT-Bench completions to the requested question order and raises a clear error if rows are missing
• writes MT-Bench run metadata, including input payload and prompt metadata, alongside saved artifacts
• lets run_mt_bench derive its default result folder and artifact name when callers do not pass them explicitly
• adds focused MT-Bench tests for both the FastChat and preset judging paths, plus coverage for completion alignment and metadata writing