Store ActorNotFound failures when store_results is set

[DSeaStar]

Fixes #448.

When a worker receives a message for an actor that is not imported locally, Results._lookup_options() treated store_results as false. after_nack then skipped the result backend, so message.get_result() waited until timeout instead of raising ActorNotFound.

The producer already knows the actor's options. before_enqueue now copies store_results / result_ttl onto the message, and _lookup_options falls back to those (then the middleware defaults) when the actor is missing. A worker that never imported the actor can still persist the failure.

Messages that never asked for results are unchanged: after_nack still no-ops when store_results is unset.