fix(draft-order): create tax-inclusive with discount (#2579)

**What**
Fix system error (500) with DraftOrder create operation when payload includes discount in a tax-inclusive context.

**How**
* Ensure newly created cart contains all required relation in order to calculate line item tax-inclusive pricing with discounts.
* Add resilience to TotalsService.getLineDiscounts()
* Ensure newly generate line items have variant relation loaded.
* fix TotalsService.getLineItemTotals to use the passed lineItem instead of relying on cartOrOrder.items.

**Test**
* Unit:
  *  TotalsService.getLineDiscounts - coverage
* Integration:
  * Admin API draft-order - coverage
  * Admin API draft-order create w/ discount in tax-inclusive

Resolves: CORE-771

Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>
Co-authored-by: Adrien de Peretti <25098370+adrien2p@users.noreply.github.com>

integration-tests/api/__tests__/admin/order/order.js

@@ -22,6 +22,7 @@ const {
   callGet,
   partial,
 } = require("../../../helpers/call-helpers")
+const { simpleShippingOptionFactory } = require("../../../factories")
 
 jest.setTimeout(30000)
 
@@ -1760,6 +1761,12 @@ describe("/admin/orders", () => {
     it("creates a swap", async () => {
       const api = useApi()
 
+      await simpleShippingOptionFactory(dbConnection, {
+        id: "testytest",
+        is_return: true,
+        region_id: "test-region",
+      })
+
       const response = await api.post(
         "/admin/orders/test-order/swaps",
         {
@@ -1770,6 +1777,10 @@ describe("/admin/orders", () => {
             },
           ],
           additional_items: [{ variant_id: "test-variant-2", quantity: 1 }],
+          return_shipping: {
+            option_id: "testytest",
+            price: 400,
+          },
         },
         {
           headers: {