Function evaluate_template_string 

pub fn evaluate_template_string(
    template: &str,
    evaluator: &Evaluator,
) -> Result<String, EngineError>

Evaluate a template string against a pre-built Evaluator.

Substitutes every @{...} segment with the evaluator’s result. Callers that need a one-shot evaluation against fresh state should pair this with build_evaluator.