fix(core-flows): item id in reservations (#9097)

**What**
- on cart completion, when creating reservations, use line item id instead of cart item id
- NOTE: inventory reservation is now done after we create the order

---

FIXES CC-448

integration-tests/modules/__tests__/cart/store/carts.spec.ts

@@ -2238,7 +2238,7 @@ medusaIntegrationTestRunner({
               location_id: stockLocation.id,
               inventory_item_id: inventoryItem.id,
               quantity: cart.items[0].quantity,
-              line_item_id: cart.items[0].id,
+              line_item_id: response.data.order.items[0].id,
             })
           )