feat(core-flows,medusa): adds inventory kit creation to variants endpoint (#7599)

what:

When creating a variant, we can now create inventory as a part of the product and variants create endpoint.

This applies only to variants where `manage_inventory=true`. 2 cases present itself:

1. When inventory_items are present
  - Link an inventory item with required_quantity to the variant
  - the inventory item already needs to be present
2. When inventory_items are not present
  - A default inventory item will be created
  - links the created item to the variant with a default required_quantity
  
  
RESOLVES CORE-2220

packages/medusa/src/api/admin/products/route.ts

@@ -59,5 +59,6 @@ export const POST = async (
     req.scope,
     req.remoteQueryConfig.fields
   )
+
   res.status(200).json({ product: remapProductResponse(product) })
 }

packages/medusa/src/api/admin/products/validators.ts

@@ -128,27 +128,37 @@ export const AdminCreateProductType = z.object({
 export type AdminCreateProductVariantType = z.infer<
   typeof AdminCreateProductVariant
 >
-export const AdminCreateProductVariant = z.object({
-  title: z.string(),
-  sku: z.string().nullable().optional(),
-  ean: z.string().nullable().optional(),
-  upc: z.string().nullable().optional(),
-  barcode: z.string().nullable().optional(),
-  hs_code: z.string().nullable().optional(),
-  mid_code: z.string().nullable().optional(),
-  allow_backorder: z.boolean().optional().default(false),
-  manage_inventory: z.boolean().optional().default(true),
-  variant_rank: z.number().optional(),
-  weight: z.number().nullable().optional(),
-  length: z.number().nullable().optional(),
-  height: z.number().nullable().optional(),
-  width: z.number().nullable().optional(),
-  origin_country: z.string().nullable().optional(),
-  material: z.string().nullable().optional(),
-  metadata: z.record(z.unknown()).optional(),
-  prices: z.array(AdminCreateVariantPrice),
-  options: z.record(z.string()).optional(),
-})
+export const AdminCreateProductVariant = z
+  .object({
+    title: z.string(),
+    sku: z.string().nullable().optional(),
+    ean: z.string().nullable().optional(),
+    upc: z.string().nullable().optional(),
+    barcode: z.string().nullable().optional(),
+    hs_code: z.string().nullable().optional(),
+    mid_code: z.string().nullable().optional(),
+    allow_backorder: z.boolean().optional().default(false),
+    manage_inventory: z.boolean().optional().default(true),
+    variant_rank: z.number().optional(),
+    weight: z.number().nullable().optional(),
+    length: z.number().nullable().optional(),
+    height: z.number().nullable().optional(),
+    width: z.number().nullable().optional(),
+    origin_country: z.string().nullable().optional(),
+    material: z.string().nullable().optional(),
+    metadata: z.record(z.unknown()).optional(),
+    prices: z.array(AdminCreateVariantPrice),
+    options: z.record(z.string()).optional(),
+    inventory_items: z
+      .array(
+        z.object({
+          inventory_item_id: z.string(),
+          required_quantity: z.number(),
+        })
+      )
+      .optional(),
+  })
+  .strict()
 
 export type AdminUpdateProductVariantType = z.infer<
   typeof AdminUpdateProductVariant

packages/medusa/src/api/store/products/helpers.ts

@@ -87,8 +87,8 @@ export const wrapVariantsWithInventoryQuantity = async (
 
     for (const link of links) {
       const requiredQuantity = link.required_quantity
-      const availableQuantity = (link.inventory.location_levels || []).reduce(
-        (sum, level) => sum + level.available_quantity || 0,
+      const availableQuantity = (link.inventory?.location_levels || []).reduce(
+        (sum, level) => sum + (level?.available_quantity || 0),
         0
       )