Add support for capturing full conversation traces during LLM generation,
enabling debugging and fine-tuning dataset creation.
Changes:
- Add `with_trace` field to LLMTextColumnConfig for per-column trace control
- Add `debug_override_save_all_column_traces` to RunConfig for global trace
- Introduce ChatMessage dataclass for structured message representation
- Update ModelFacade.generate() to return full message trace
- Rename trace column postfix from `__reasoning_trace` to `__trace`
- Add comprehensive traces documentation
Traces capture system/user/assistant messages in order, enabling visibility
into the full generation conversation including correction retries.
