Create Shop Pay Truemed Session Refund

Record a refund you already issued on your own payment rail. `refund_amount_cents` is the authoritative refunded amount — it alone moves the reconciliation numbers. `refund_line_items` describes this refund's own return lines: the units returned and their economic value (post-discount subtotal and tax). The line values are facts about what came back, not an allocation of the money — a partial-money refund against returned lines worth more is legal, and the list may be empty or omitted for refunds with no returned units (shipping, adjustments, goodwill). Truemed updates the session history, the reconciliation reads, and the returned-units ledger to match. The call is idempotent on `idempotency_key`. Replaying the same refund with identical contents returns the result recorded the first time; a replay whose contents differ is rejected with a `400` rather than merged. Line order doesn't matter, so the same lines in a different order count as the same refund.