Fix `MeanAverageRecall` compute mAR@K using top-K detections per image [COCO-compliant] (#2136)

* Supersedes #1967
* fix: COCO-compliant mAR calculation
* Add complex test of mAP
* fix(metrics): cast optional detections fields in mAR metric for mypy
* Add `create_yolo_dataset` utility and refactor tests with fixtures for reusable scenarios
* Refine docstrings for clarity and consistency, adding inline formatting and fixing typos in test helpers and metrics.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: jirka <6035284+Borda@users.noreply.github.com>