chore: remove unused fine-tuning data validators (#3863)

Removes the unused fine-tuning data validators left behind when the
legacy CLI was removed. Deletes 809 lines; there are no remaining
callers or associated tests.

Validation: 217 tests passed, 2 skipped; all 13,122 tests collect. Ruff
lint, mypy, SDK imports, and wheel/sdist builds pass using the existing
frozen dependencies. The format check flags two unchanged files.