Create Shop Pay Truemed Session Refund

Record a refund you already issued on your own payment rail. `refund_amount_cents` is the sole authoritative refunded amount — it alone moves the reconciliation numbers. `refund_line_items` is optional and is used only to classify how much of the refund is HSA/FSA-eligible: each line names which item came back and how many units, so Truemed knows which portion of the refunded money was eligible. It is never a money allocation and is never reconciled against `refund_amount_cents`. Omit it for amount-only refunds and Truemed falls back to deriving the eligible/ineligible split from the session's captured composition. A refund cannot exceed the session's captured amount net of prior refunds. The call is idempotent on `idempotency_key`. The replay identity includes `refund_line_items` when present: replaying the same refund with an identical amount and identical lines returns the result recorded the first time, while a replay whose amount OR lines differ is rejected with a `400` rather than merged. Line order does not matter.