Guard against missing payments in the refund tool

[sentry]

Fixes a TypeError in /api/chat when attempting to refund orders placed prior to the payments launch.

• Update selectPayment return type to Promise<Payment | undefined> and remove the unsafe cast.
• Add a null check on payment in refundOrder before reading payment.chargeId, gracefully returning an unrefunded status if no payment row exists.

Fixes DEBUGGING-AGENTS-STOREFRONT-6

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