Given a customers DataFrame with country (sometimes missing) and billing_country, resolve each customer's country: use country when present, otherwise fall back to billing_country.
Return the resolved values as a Python list, in row order; if both columns are missing, the resolved value is None. Don't modify the input DataFrame.
What the tests check
3 on Run · 4 on Submit01falls back to billing_country when country is missing
02keeps country when present
03the input DataFrame is left unchanged
Submit runs these and one more check you don’t see, so code written for the visible tests alone can still miss.