Add refinement to ensure metered line items have zero cost

The billing schema has been updated to include a refinement that ensures metered line items have a cost of zero. To clarify, a different line item type should be added for a flat fee in Stripe. Additionally, unnecessary verifications and logic regarding item costs have been removed from the PlanPicker and LineItemDetails components.

packages/billing/gateway/src/components/line-item-details.tsx

@@ -187,10 +187,6 @@ export function LineItemDetails(
             return <PerSeat key={item.id} />;
 
           case 'metered': {
-            if (item.cost > 0) {
-              return <FlatFee key={item.id} />;
-            }
-
             return <Metered key={item.id} />;
           }
         }