Fix 7846: add_column and add_item erroneously(?) require new_fingerprint parameter 
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Summary of the change:

Made new_fingerprint optional (Optional[str] = None) in Dataset.add_column and Dataset.add_item
Added a simple test to verify both methods work without providing a fingerprint

Why this change is safe:

The Dataset constructor already handles fingerprint=None by generating a new fingerprint automatically
No internal logic is broken — if a user provides a fingerprint, it’s still used as before
The change only affects the function signature, making it more user-friendly without changing any functionality