Get Billing Balance Endpoint

Return your organization's effective credit price for every purchasable action. Prices are resolved with the same precedence billing uses at charge time (subscription override → partner plan → Base plan → list price), so each item's `credits` is exactly what a call would cost you right now; `priceSource` says which tier the price came from. Each item also carries `enabled`: whether your organization can perform the action today given platform availability and your entitlements — a priced but disabled item shows what it would cost once enabled. `pricingMode` tells you the price shape: `fixed` items charge the flat `credits` amount every call; `free` items are never charged (`credits` is 0); `variable` items (e.g. a registry document whose fee depends on the document) have no single guaranteed price — `credits` is null and `estimatedCredits` gives a baseline so you can still anticipate cost. Shareholder-graph extensions (`perUnit` = "per extended node") are charged once per extended frontier node, against that node's jurisdiction's extension SKU — extending a graph across a border bills each node at its price configured for its own jurisdiction.