fix(yolo): emit accurate dataset YAML

[amanharshx]

Summary

Correct YOLO dataset metadata generation for detection and classification exports.

Motivation

Generated YAML referenced nonexistent splits and was invalid for folder-based classification datasets.

Key Changes

• Emit only splits represented in final ZIP.
• Warn when detection data lacks required train or val split.
• Omit data.yaml for classification exports.
• Add regression coverage for split combinations, failed downloads, labels-only exports, and classification.
• Verified with Cargo tests, Clippy, formatting, Rust build, Bun tests, and frontend build.
• Sanity check: source splits → expected YAML keys → actual keys matched across all tested combinations.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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