Function validate_preset_for_exec 

pub fn validate_preset_for_exec(
    preset_name: &str,
    spec: &CommandSpec,
    schema: &Schema,
) -> Result<(), String>

Validate a single preset that’s about to be invoked. Combines the always-on choices: check and, if schema.strict, the unknown-key rejection — scoped to just this preset, not the whole commands: map. Called from the exec path so typos in a sibling preset don’t block --help for a correct one.