Migrate ConversationManager to call apply_management via hooks

[zastrowm]

In PR#1374, we implemented the ability for conversation-managers to register hooks, so that they can apply conversation management in more places.

Long term this also unlocks the ability to be more losely coupled from the agent-loop - specifically, the base ConversationManager class could register callbacks so that apply_management and reduce_context are invoked via hooks instead of being hard-coded in the agent-loop.

Also, what is the story for transitioning from "manual" conversation management, meaning conversation_manager.apply_management, to using only hooks.

Originally posted by @dbschmigelski in #1374 (comment)

To do this, we'd need to do two things:

1. Allow hooks to intercept model (ContextOverflow) exceptions and retry model invocation

2. Update ConversationManager to subscribe to (2) and apply_management that way

For backwards compatability, we would still need to expose apply_management and reduce_context as public methods, but we could still decouple it from the agent-loop.