fix(medusa): calculates correct taxes and totals on order with gift cards (#1807)

**What**
Since the release of the Tax API the line item totals calculations on orders with gift cards have been wrong. To understand the bug consider the below order:

Region:
- tax_rate: 25%
- gift_cards_taxable: true

Order:
- applied gift card: 1000
- items: 
  - A: unit_price: 1000
  - B: unit_price: 500
- Subtotal: 1500

**Previous calculation method**
1. Determine how much of the gift card is used for each item using `item_total / subtotal * gift_card_amount`:
  - Item A: 1000/1500 * 1000 = 666.67
  - Item B: 500/1500 * 1000 = 333.33
2. Calculate line item totals including taxes using `(unit_price - gift_card) * (1 + tax_rate)`
  - Item A: 1000 - 666.67 = 333.33; vat amount -> 83.33
  - Item B: 500 - 333.33 = 166.67; vat amount -> 41.67
3. Add up the line item totals: order subtotal = 500; vat amount = 125; total = 625

This is all correct at the totals level; but at the line item level we should still use the "original prices" i.e. the line item total for item a should be (1000 * 1.25) = 1250 with a tax amount of 250. 

**New calculation method**
1. Use default totals calculations
  - Item A: subtotal: 1000, tax_total: 250, total: 1250
  - Item B: subtotal: 500, tax_total: 125, total: 625
2. Add up the line item totals: subtotal: 1500, tax_total: 375, total: 1875
3. Reduce total with gift card: subtotal: 1500 - 1000 = 500, tax_total: 375 - 250 = 125, total = 625

Totals can now be forwarded correctly to accounting plugins.

Fixes CORE-310.

integration-tests/api/factories/index.ts

@@ -1,3 +1,4 @@
+export * from "./simple-gift-card-factory"
 export * from "./simple-payment-factory"
 export * from "./simple-batch-job-factory"
 export * from "./simple-discount-factory"

integration-tests/api/factories/simple-gift-card-factory.ts

@@ -0,0 +1,33 @@
+import { GiftCard } from "@medusajs/medusa"
+import faker from "faker"
+import { Connection } from "typeorm"
+
+export type GiftCardFactoryData = {
+  id?: string
+  code?: string
+  region_id: string
+  value: number
+  balance: number
+}
+
+export const simpleGiftCardFactory = async (
+  connection: Connection,
+  data: GiftCardFactoryData,
+  seed?: number
+): Promise<GiftCard> => {
+  if (typeof seed !== "undefined") {
+    faker.seed(seed)
+  }
+
+  const manager = connection.manager
+
+  const toSave = manager.create(GiftCard, {
+    id: data.id,
+    code: data.code ?? "TESTGCCODE",
+    region_id: data.region_id,
+    value: data.value,
+    balance: data.balance,
+  })
+
+  return await manager.save(toSave)
+}

integration-tests/api/factories/simple-product-factory.ts

@@ -112,5 +112,9 @@ export const simpleProductFactory = async (
     await simpleProductVariantFactory(connection, factoryData)
   }
 
-  return manager.findOne(Product, { id: prodId }, { relations: ["tags", "variants", "variants.prices"] })
+  return manager.findOne(
+    Product,
+    { id: prodId },
+    { relations: ["tags", "variants", "variants.prices"] }
+  )
 }

integration-tests/api/factories/simple-region-factory.ts

@@ -9,6 +9,7 @@ export type RegionFactoryData = {
   tax_rate?: number
   countries?: string[]
   automatic_taxes?: boolean
+  gift_cards_taxable?: boolean
 }
 
 export const simpleRegionFactory = async (
@@ -29,6 +30,7 @@ export const simpleRegionFactory = async (
     currency_code: data.currency_code || "usd",
     tax_rate: data.tax_rate || 0,
     payment_providers: [{ id: "test-pay" }],
+    gift_cards_taxable: data.gift_cards_taxable ?? true,
     automatic_taxes:
       typeof data.automatic_taxes !== "undefined" ? data.automatic_taxes : true,
   })