- Replace direct field access with private attributes and property decorators
- Implement deep copy protection for mutable objects (dict, LLMUsage)
- Add helper methods: set_output(), get_output(), update_outputs()
- Add increment_node_run_steps() and add_tokens() convenience methods
- Update loop_node and event_handlers to use new accessor methods
- Add comprehensive unit tests for immutability and validation
- Ensure backward compatibility with existing property access patterns