move sanitize_metadata out of save_metadata

[snimu]

Description

Moves sanitize_metadata out of save_metadata. The reason is that there's no reason for the function to be defined inside the other function, and that prime-rl tries to import sanitize_metadata which it cannot if it's nested in such a way.

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

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Note

Low Risk
Small refactor that only changes function scope/visibility; serialization and file-writing behavior remains the same.

Overview
Moves sanitize_metadata out of save_metadata in verifiers/utils/save_utils.py, making it a top-level helper that can be imported by other packages.

save_metadata now calls the extracted sanitize_metadata function, preserving the same metadata filtering (path_to_save, date) before writing metadata.json.

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