payOnce reserved MAX_AMOUNT before awaiting confirmation but only released
it when confirmWithUser returned false. A prompt that threw (timeout, closed
window) or a challenge the payment policy rejected before signing kept the
reservation, so five such failures exhausted SESSION_CAP with zero payments.
Track per call whether a request carrying PAYMENT-SIGNATURE / X-PAYMENT was
actually sent, and release the reservation in a finally block unless it was.
A sent signed request may settle, so it stays counted even if the response
fails. The reservation still happens before any await, so concurrent calls
cannot overrun the cap. Mirrored in ja-JP and zh-CN.
Notorious D.E.V
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