Function deserialize_payload 

pub fn deserialize_payload(
    hook_name: &str,
    value: Value,
) -> Result<Box<dyn PluginPayload>, HostError>

Rebuild a typed payload from JSON, per the hook’s payload kind.

Used for a modified_payload coming back from the worker: it must be re-erased as the same concrete type the executor expects for the hook, or a downstream downcast fails and the modification is dropped.