fix ocrbench v2 scoring regressions

[Luodian]

Summary

• Fix OCRBench v2 chart parsing so string ground truth is parsed from the answer instead of the prediction.
• Make OCRBench v2 aggregation stateless across repeated runs in the same Python process.
• Isolate text spotting temporary artifacts per call and remove fragile OCRBench v2 runtime debug leftovers.

In scope

• Fix chart parsing en scoring in lmms_eval/tasks/ocrbench_v2/utils.py.
• Replace module-level OCRBench v2 score accumulation with per-run aggregation buckets.
• Move spotting metric scratch files to a temporary work directory.
• Fix scalar-answer fallback paths in lmms_eval/tasks/ocrbench_v2/vqa_metric.py.
• Add OCRBench v2 regression tests in test/eval/test_ocrbench_v2.py.

Out of scope

• Changes to non-OCRBench tasks or evaluator-wide scoring behavior.
• Benchmark prompt changes or dataset updates.

Validation

• uv run python -m pytest test/eval/test_ocrbench_v2.py -q | sample size: N=5 tests | key metrics: 5 passed | result: pass
• uv run pre-commit run --all-files | sample size: N=all tracked files | key metrics: black, isort passed | result: pass

Risk / Compatibility

• Low risk: changes are scoped to OCRBench v2 evaluation helpers and covered by task-specific regression tests.
• Behavior change is intentional for previously incorrect OCRBench v2 scores and repeated-run aggregation.

Type of Change

• Bug fix (non-breaking change)
• New feature
• New benchmark/task
• New model integration
• Breaking change
• Documentation update
• Refactoring (no functional changes)