fix: save updated task result to output file

[Pasa-Juliano]

Fixes bug where only pre-guardrail task results were being saved to the output file.

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Note

Fixes output persistence of final task results

• When output_file is set, save from task_output.json_dict → task_output.pydantic.model_dump_json() → task_output.raw, ensuring the final post-guardrail result is written (applies to both execute and aexecute paths)
• Minor no-op formatting in _execute_task_async

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[Pasa-Juliano]

Solves #4156