Fix BillingSection test mock types so beta build compiles

Default subscription mock inferred plan as null; widen to union so
foundersSubscription can be passed. Unblocks CRA production build.

llm-fe/src/llm-fe/components/BillingSection/BillingSection.test.tsx

@@ -114,10 +114,16 @@ const emptySubscription = {
   },
 };
 
+type SubscriptionMock = typeof emptySubscription | typeof foundersSubscription;
+
 const mockFinanceGets = ({
   invoices = [] as unknown[],
   payments = [] as unknown[],
-  subscription = emptySubscription,
+  subscription = emptySubscription as SubscriptionMock,
+}: {
+  invoices?: unknown[];
+  payments?: unknown[];
+  subscription?: SubscriptionMock;
 } = {}) => {
   mockGet.mockImplementation((url: string) => {
     if (url === '/finance/invoices/') {