Fix callback persistence chaining using Fanout pattern

[kieranklaassen]

Problem

When using the Rails integration (acts_as_chat), persistence callbacks are set up on the underlying RubyLLM::Chat instance. However, because the original implementation of @on callbacks in Chat simply replaced the existing proc, users setting their own callbacks (e.g., chat.to_llm.on_new_message { ... }) would accidentally overwrite and remove the persistence logic. This resulted in lost chat history.

Solution

This PR implements a Callback Fanout pattern directly in RubyLLM::Chat.

1. CallbackFanout: A new internal class that acts like a hash but stores an array of callables for each key. When invoked, it calls all registered callbacks in order.
2. Simplified Integration: The ActiveRecord integration code (ChatMethods and ActsAsLegacy) was previously trying to manually wrap and chain callbacks to prevent this issue. This brittle logic has been removed in favor of the robust native support in Chat.
3. Behavior: Now, calling on_new_message (and others) appends the new callback rather than replacing the existing one. This ensures that both system callbacks (persistence) and user callbacks run successfully.

Testing

• Added a regression test in spec/ruby_llm/active_record/acts_as_spec.rb specifically checking that chaining callbacks on to_llm preserves persistence.
• Validated that existing tests continue to pass.