hf_hub validate_readme reports a card with unparseable front matter as valid #5232

DatasetCardValidator::validate_readme (crates/aprender-data/src/hf_hub/validation.rs) returns an empty error list whenever the README front matter fails to parse as YAML:

let Ok(yaml) = serde_saphyr::from_str_with_options::<serde_json::Value>(...) else {
    return errors;
};

A card with broken YAML front matter therefore validates as clean, and validate_readme_strict returns Ok(()). The skip predates the YAML-reader swap (#5205): on main, under serde_yaml, the code was the same.

Expected: an unparseable front matter is reported, either as a ValidationError on a frontmatter field or as an Err from validate_readme_strict, never as a pass.

RED row to add with the fix: a README whose front matter is task_categories: [unclosed must produce at least one error.

Refs #5205