warn: ignore system prompt/few-shot when prompt is present

[rasdani]

Description

warn when provided system prompt or few-shot example will be ignored because prompt column being already present.
especially helpful when loading existing upstream datasets which do prompt column differently.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Adds guardrails for pre-formatted datasets.

• In Environment._ensure_prompt, when a prompt column already exists, the provided system_prompt and few_shot are ignored and a warning is logged to inform the user

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