Add Multi‑Value Discount Filters & Billing Error Type

Removed the limitation of single‑value filters on the discountNodes and discountNodesCount queries by allowing comma‑separated values for fields such as combines_with, discount_class, discount_type, method, status, and type. The query description has been expanded to clarify the syntax and usage of these multi‑value filters. A new type, SubscriptionBillingAttemptUnexpectedError, has been introduced to surface unexpected errors during subscription billing attempts. The Publication type now exposes a new channels connection field to list channels attached to a publication. Fields kana and kanji in ShopifyPaymentsJpChargeStatementDescriptor have been marked deprecated and will be removed in a future release. The enum SubscriptionBillingAttemptGeneralErrorCode has had the value UNEXPECTED_ERROR removed in favour of the new error type.