fix(inventory): serialize variant locks using resolved variant ID to prevent double-booking

[Pratyush-Panda-2006]

Description

Fixes race condition and double-booking in \�ackend/services/inventoryReservationService.js. Previously, when an order line with color/size was submitted without explicit \�ariantId,
esolveLockVariant\ found the variant record but \line.variantId\ was left as \

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