Add support for custom billing plans

This commit includes updates to the billing schema to allow for the creation of custom billing plans. These types of plans have no line items associated with them. Changes have also been made to properly handle the absence of primary line items in such plans.

packages/billing/core/src/create-billing-schema.ts

@@ -87,12 +87,17 @@ export const PlanSchema = z
       })
       .min(1),
     interval: BillingIntervalSchema.optional(),
+    custom: z.boolean().default(false).optional(),
+    label: z.string().min(1).optional(),
+    href: z.string().min(1).optional(),
     lineItems: z.array(LineItemSchema).refine(
       (schema) => {
         const types = schema.map((item) => item.type);
+
         const perSeat = types.filter(
           (type) => type === LineItemType.PerSeat,
         ).length;
+
         const flat = types.filter((type) => type === LineItemType.Flat).length;
 
         return perSeat <= 1 && flat <= 1;
@@ -111,10 +116,32 @@ export const PlanSchema = z
       .optional(),
     paymentType: PaymentTypeSchema,
   })
-  .refine((data) => data.lineItems.length > 0, {
-    message: 'Plans must have at least one line item',
-    path: ['lineItems'],
-  })
+  .refine(
+    (data) => {
+      if (data.custom) {
+        return data.lineItems.length === 0;
+      }
+
+      return data.lineItems.length > 0;
+    },
+    {
+      message: 'Non-Custom Plans must have at least one line item',
+      path: ['lineItems'],
+    },
+  )
+  .refine(
+    (data) => {
+      if (data.custom) {
+        return data.lineItems.length === 0;
+      }
+
+      return data.lineItems.length > 0;
+    },
+    {
+      message: 'Custom Plans must have 0 line items',
+      path: ['lineItems'],
+    },
+  )
   .refine(
     (data) => data.paymentType !== 'one-time' || data.interval === undefined,
     {