**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>
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