[Benchmark] Add ActiveVision benchmark

[lzpp2598]

Description

Adds ActiveVision (arXiv:2607.16165), an exam for active observers: 85 single-image QA items across 17 tasks / 3 categories, rule-based exact-match scoring.

• Dataset class: ActiveVisionDataset (TYPE='VQA') in vlmeval/dataset/activevision.py
• Data: auto-downloaded from HF activevisionai/ActiveVision (metadata.jsonl + images), converted to a cached TSV with base64-embedded images on first run
• Prompt: question text is sent verbatim (it already instructs the model to answer in <answer>...</answer> tags), no extra instruction appended
• Scoring: faithful port of the official eval/lib/scoring.py — answer-tag extraction, int/string normalisation, exact match; reports Overall / per-category / per-task accuracy

Validation

• Smoke-tested: build_dataset('ActiveVision') loads 85 samples; build_prompt yields image+text messages; evaluate on synthetic predictions produces the expected Overall / 3 category / 17 task columns
• Scoring functions verified against the official repo's implementation