ci: add real-data task input A/B snapshot gate

[Luodian]

Summary

• add a request-boundary snapshot checker (tools/task_input_capture.py) that loads representative real tasks and compares base/head request payload boundaries without serializing image/video content
• add real-task capture spec coverage for MMMU + MMLU + MMLU loglikelihood (test/eval/task_input_specs/redundancy_refactor.yaml)
• add a dedicated CI workflow (.github/workflows/task-input-ab.yml) to capture base/head snapshots, compare JSON output, and upload artifacts on mismatch

What changed in CI

• this gate checks request concatenation and answer-format-sensitive boundaries from real datasets
• chat task coverage (mmmu_val) uses text_and_structure mode to verify message text + role/content-type structure while excluding media payloads
• MMLU paths use request_only mode to verify ctx + continuation boundary stability
• pull_request runs pin checker/spec from the base revision to avoid mutable-comparator false negatives
• workflow now triggers on both lmms_eval/tasks/** and lmms_eval/api/** changes

Why

• protect refactors from silently changing model-facing request formatting
• keep CI representative and useful (real data) while avoiding expensive/noisy image/video payload comparisons