fix(product): delete individual variant metadata fields (#16179)

## Summary

**What:** Fix variant metadata updates so an individual field can be deleted without removing or overwriting the remaining fields.

**Why:** `ProductModuleService.updateVariants_` uses `upsertWithReplace`, which bypassed the `mergeMetadata` behavior used by other metadata update paths. As a result, an empty-string value was persisted instead of deleting the key.

**How:** Merge incoming variant metadata with the persisted metadata before calling `upsertWithReplace`. This preserves unspecified keys and removes keys whose incoming value is an empty string. The change includes unit and integration regression coverage plus a patch changeset.

**Testing**

- Product module TypeScript build passes.
- Product module unit suite passes: 2 tests, 2 passed.
- Targeted ESLint checks pass for the implementation and tests.
- The new tests and changeset pass Prettier; `git diff --check` passes.
- Added PostgreSQL-backed integration coverage for the exact multi-field deletion scenario; it will run in CI because PostgreSQL is not available in the local environment.

---

## Examples

```ts
await productModuleService.updateProductVariants(variant.id, {
  metadata: {
    obsolete: "",
  },
})
```

The `obsolete` key is removed while all other metadata fields remain unchanged.

---

## Checklist

- [x] I have added a **changeset** for this PR
- [x] The changes are covered by relevant **tests**
- [x] I have verified the code works as intended locally
- [x] I have linked the related issue(s) if applicable

---

## Additional Context

Fixes #16163.

Author: ihopenre-eng

.changeset/friendly-chairs-fix.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/product": patch
+---
+
+fix(product): allow deleting individual variant metadata fields

packages/modules/product/integration-tests/__tests__/product-module-service/product-variants.spec.ts

@@ -444,6 +444,29 @@ moduleIntegrationTestRunner<IProductModuleService>({
           expect(productVariant.title).toEqual("new test")
         })
 
+        it("should delete one metadata field without removing the others", async () => {
+          await service.updateProductVariants(variantOne.id, {
+            metadata: {
+              keep: "value",
+              remove: "value",
+            },
+          })
+
+          await service.updateProductVariants(variantOne.id, {
+            metadata: {
+              remove: "",
+            },
+          })
+
+          const productVariant = await service.retrieveProductVariant(
+            variantOne.id
+          )
+
+          expect(productVariant.metadata).toEqual({
+            keep: "value",
+          })
+        })
+
         it("should update scalar fields without altering the variant's options", async () => {
           // A scalar-only update (no `options` in the payload) skips the option
           // resolution/uniqueness/relation-reconcile path. This must NOT wipe or

packages/modules/product/src/services/__tests__/product-module-service.spec.ts

@@ -0,0 +1,58 @@
+import ProductModuleService from "../product-module-service"
+
+describe("ProductModuleService", () => {
+  it("should merge variant metadata before replacing the entity", async () => {
+    const productVariantService = {
+      list: jest.fn().mockResolvedValue([
+        {
+          id: "variant_1",
+          product_id: "product_1",
+          metadata: {
+            keep: "value",
+            remove: "value",
+          },
+        },
+      ]),
+      upsertWithReplace: jest.fn().mockImplementation(async (input) => ({
+        entities: input,
+      })),
+    }
+    const service = new ProductModuleService(
+      {
+        productVariantService,
+      } as any,
+      {} as any
+    )
+    const context = {
+      transactionManager: {},
+    }
+
+    await (service as any).updateVariants_(
+      [
+        {
+          id: "variant_1",
+          metadata: {
+            remove: "",
+          },
+        },
+      ],
+      context
+    )
+
+    expect(productVariantService.upsertWithReplace).toHaveBeenCalledWith(
+      [
+        {
+          id: "variant_1",
+          product_id: "product_1",
+          metadata: {
+            keep: "value",
+          },
+        },
+      ],
+      {
+        relations: [],
+      },
+      context
+    )
+  })
+})

packages/modules/product/src/services/product-module-service.ts

@@ -44,6 +44,7 @@ import {
   MedusaContext,
   MedusaError,
   MedusaService,
+  mergeMetadata,
   MessageAggregator,
   Modules,
   partitionArray,
@@ -659,11 +660,23 @@ export default class ProductModuleService
 
     // Data normalization
     const variantsWithProductId: UpdateProductVariantInput[] = variants.map(
-      (v) => ({
-        ...data.find((d) => d.id === v.id),
-        id: v.id,
-        product_id: v.product_id,
-      })
+      (variant) => {
+        const update = data.find(({ id }) => id === variant.id)!
+
+        return {
+          ...update,
+          id: variant.id,
+          product_id: variant.product_id,
+          ...(isPresent(update.metadata)
+            ? {
+                metadata: mergeMetadata(
+                  variant.metadata ?? {},
+                  update.metadata!
+                ),
+              }
+            : {}),
+        }
+      }
     )
 
     let productVariantsWithOptions: UpdateProductVariantInput[] =