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>

packages/medusa/src/services/__tests__/line-item.js

@@ -413,6 +413,14 @@ describe("LineItemService", () => {
           metadata: {},
           should_merge: true,
           includes_tax: true,
+          variant: expect.objectContaining({
+            id: expect.any(String),
+            product: expect.objectContaining({
+              thumbnail: "",
+              title: "Test product",
+            }),
+            title: "Test variant",
+          }),
         })
       })
     })
@@ -524,6 +532,14 @@ describe("LineItemService", () => {
           metadata: {},
           should_merge: true,
           includes_tax: false,
+          variant: expect.objectContaining({
+            id: expect.any(String),
+            product: expect.objectContaining({
+              thumbnail: "",
+              title: "Test product",
+            }),
+            title: "Test variant",
+          }),
         })
       })
     })