BaseModel.__init__ silently swallows unknown keyword arguments #17

BaseModel.__init__ takes **kwargs and discards them without a word, while Error.__init__ and ErrorResponse.__init__ in the same package both logger.warning about leftovers. So every model silently accepts any keyword it does not know.

Measured:

>>> item = BasketItem(title="t", type="goods", quantitiy=3)
>>> item.kind, item.serialize()["type"], item.serialize()["quantity"]
(None, '', None)

type is the name the API itself uses and the name serialize() emits, so it is the first thing a reader reaches for — and quantitiy is an ordinary typo that drops a quantity out of a payment payload.

This is not hypothetical: the SDK's own sandbox tests passed type="goods" and were therefore sending an empty item type for as long as they existed. #13 corrected the two call sites in tests/, but examples/03_installment_plans.py:57 still has it — the one public example of building a basket item, i.e. what a downstream user copies:

amountPerUnitGross=AMOUNT, basketItemReferenceId="item-1", type="goods",

Two things worth doing:

1. Fix the example.
2. Mirror Error's behaviour in BaseModel.__init__ — a logger.warning on leftover kwargs. It needs BasketItem.fromDict and Basket.fromDict to pop the keys they deliberately pass through (type, id), which is a small change and makes the round trip explicit rather than accidental.

AGENTS.md announces a camelCase → snake_case rename for 2.0, which will turn every unported keyword into a silently empty field. A warning now is cheap insurance for that.