Implement an Extensible Chatbot App

Implement a `ChatApp` class that processes user messages and bot responses. The system should support multiple bot types. The design must be extensible: adding a new bot type should not require modifying existing `ChatApp` logic. Requirements: - A user can send a message to the chat app. - The app dispatches the message to one or more registered bots. - Each bot can decide whether and how to respond. - The app records the message log. - The design should support adding new bot implementations through registration or dependency injection. Follow-up: Extend the design to support multiple channels. Each channel must have independent state, including its own message log and bot state.