Function validate_yaml_with_prefix 

pub fn validate_yaml_with_prefix(
    input: &str,
    prefix: &str,
) -> Option<YamlDiagnostic>

Structural validation for embedded (prefixed) YAML. Strips the per-line prefix to the plain-YAML baseline and runs the standard [super::validator::validate_yaml] pass. The verdict matches the stripped baseline; diagnostic offsets refer to the stripped text (host-offset remapping is a later concern). An empty prefix is plain validation.