Fix the idempotent charge handler for duplicate requests

Your payment API takes an idempotency key with each create-charge request, so a client that retries never pays twice. Twice an agent wrote code that passed the happy path, yet a duplicate request still created a second charge. You model the endpoint over a stream of requests to pin down what it must do.

An assistant wrote resolve_idempotent_charges(request_keys). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• request_keys: the idempotency key of each request, in arrival order.
• Return, for each request, the id of the charge it gets back, or 0 when it is refused.

Rules the tests check:

• Trim whitespace from both ends of each key. Keys then compare exactly, so A and a are different keys.
• A key that is empty after trimming creates no charge and gets 0.
• The first request with a key creates a new charge. Charge ids are 1, 2, 3 and so on, in the order charges are created.
• A later request with the same key creates nothing and gets the id of the charge that key created.