Fix callback persistence chaining using Fanout pattern

[kieranklaassen]

Problem

When using acts_as_chat, persistence callbacks were silently overwritten when users added their own callbacks:

chat = Chat.create!(model: 'gpt-4')
chat.to_llm.on_new_message { puts "my callback" }  # This overwrote persistence!
chat.ask("Hello")  # Messages not saved to database

Solution

Add a simple CallbackFanout class that appends callbacks instead of replacing them:

# Now both callbacks run
chat.to_llm.on_new_message { puts "my callback" }
chat.ask("Hello")  # Messages saved AND user callback runs

Changes

• lib/ruby_llm/chat.rb: Add 18-line CallbackFanout class
• lib/ruby_llm/active_record/*.rb: Remove 32 lines of brittle callback wrapping code

Net result: -20 lines while fixing the bug.