Function evaluate_assertion 

pub fn evaluate_assertion(
    registry: &dyn PluginRegistry,
    assertion: &str,
    response: &Value,
    headers: Option<&HashMap<String, String>>,
    trailers: Option<&HashMap<String, String>>,
    timing: Option<&AssertionTiming>,
    variables: &HashMap<String, Value>,
) -> Result<Option<AssertionResult>>

Evaluate an assertion expression. Returns Ok(Some(result)) when the AST engine handled the expression, Ok(None) when the expression should fall through to the JQ evaluator.