Collect all the errors

Implement validate_user(payload) returning (clean, errors): - name: required non-empty string → stored stripped - email: required string containing '@' → stored stripped and lowercased - age: int 0..150 — a digit-string like '42' is coerced to int first (bools are NOT ints here) - every other key in the payload produces an {'field': k, 'detail': 'unknown field'} error, sorted by field name - each error is {'field': ..., 'detail': ...}; when errors is non-empty, clean may only contain fields that passed The final error list is sorted by field name. The contract: report ALL problems in one call (collect, don't abort at the first).

Difficulty: advanced