Function apply_event_result 

pub fn apply_event_result(
    processor: &mut MessageProcessor,
    result: EventResult,
) -> Option<String>

Apply an EventResult produced by a component to the processor’s state.

Returns Some(path) only for an EventResult::Action that expects a response — the path is where the eventual server response value should be written — so the caller can drive the action-response cycle. Every other variant returns None.