Function
handle-message
Callback invoked when a message is received from a subscription.
As an async func this may await other operations — including
consumer.publish to reply on msg.reply-to — while the host continues to
deliver further messages to the same instance concurrently. The message
body is a stream<u8> the handler drains, or drops to discard the payload.
The error is a per-delivery disposition (handle-message-error), not the
broker error — a handler cannot raise transport failures about a message
it has already received; what it reports is whether redelivery could help.
handle-message: async func(msg: broker-message) -> result<_, handle-message-error>;