Materialize prompt before pydantic validation

[mikasenghaas]

Description

Fixes #432.

uv run vf-eval hle -n 1 -r 1 -t 1

On current main there is no prompt (this is due to messages_to_printable, otherwise it would be this weird Pydantic iterator validation)

On this PR we get the prompt as expected.

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