Notification service with several bots per channel

Design the classes and interfaces for a notification service. A request names its recipients, the content and any of three channels: email, SMS and Slack. Each channel can have several bot integrations registered, and a request goes to every bot of every channel it selects.

Cover:

• how channels and bots are registered and how new ones are added without touching dispatch code;
• one bot failing without stopping the others, and a per-channel, per-bot result (success, failure or unknown) returned to the caller;
• idempotency: resending the same request ID returns the earlier result without calling bots again, and reusing an ID with a different payload is rejected.

Follow-up: how the design changes when delivery becomes asynchronous and individual bots are retried.