fix(utils): assign per-adjustment subtotal/total instead of cumulative sum (#16013)

## Summary

**What** — Fixes `calculateAdjustmentTotal` in `packages/core/utils/src/totals/adjustment/index.ts` so each adjustment's `subtotal`/`total` is set from its own per-adjustment value instead of the running cumulative sum.

**Why** — The loop assigned the plural running accumulators (`adjustmentsSubtotal`/`adjustmentsTotal`) onto each adjustment's own `subtotal`/`total`. Every adjustment after the first was inflated to the cumulative total. The bug is masked when a line has a single adjustment. The sibling `calculateCreditLinesTotal` (credit-lines/index.ts) already does this correctly using the per-item value, confirming intent. Aggregate return values were unaffected — only the per-adjustment object mutation was wrong.

**How** — Changed the two assignments to use the singular locals `adjustmentSubtotal` / `adjustmentTotal` computed earlier in the same loop iteration.

**Testing** — Added `packages/core/utils/src/totals/adjustment/__tests__/index.spec.ts` (no-tax and with-tax cases, asserting per-adjustment values are not cumulative). `yarn workspace @medusajs/utils test adjustment` passes.

---

## Examples

```ts
import { calculateAdjustmentTotal } from "@medusajs/utils"

const adjustments = [{ amount: 10 }, { amount: 20 }, { amount: 5 }]
calculateAdjustmentTotal({ adjustments })

adjustments.map((a) => Number(a.subtotal))
// before: [10, 30, 35]   after: [10, 20, 5]

Author: Venkat-jaswanth

.changeset/fix-adjustment-cumulative-total.md

@@ -0,0 +1,7 @@
+---
+"@medusajs/utils": patch
+---
+
+fix(utils): assign per-adjustment subtotal/total instead of the cumulative running sum
+
+`calculateAdjustmentTotal` wrote the plural running accumulators (`adjustmentsSubtotal`/`adjustmentsTotal`) onto each adjustment's own `subtotal`/`total`, instead of the singular per-adjustment values. Every adjustment after the first was inflated to the cumulative total; the bug was masked for lines with a single adjustment. The aggregate return values were unaffected.

packages/core/utils/src/totals/adjustment/__tests__/index.spec.ts

@@ -0,0 +1,37 @@
+import { calculateAdjustmentTotal } from "../index"
+
+describe("calculateAdjustmentTotal", function () {
+  it("assigns each adjustment its own subtotal/total, not the running cumulative sum", function () {
+    const adjustments: any[] = [{ amount: 10 }, { amount: 20 }, { amount: 5 }]
+
+    const result = calculateAdjustmentTotal({ adjustments })
+
+    // Per-adjustment fields must reflect each adjustment individually.
+    expect(Number(adjustments[0].subtotal)).toEqual(10)
+    expect(Number(adjustments[0].total)).toEqual(10)
+    // Regression: these were 30 and 35 (cumulative) before the fix.
+    expect(Number(adjustments[1].subtotal)).toEqual(20)
+    expect(Number(adjustments[1].total)).toEqual(20)
+    expect(Number(adjustments[2].subtotal)).toEqual(5)
+    expect(Number(adjustments[2].total)).toEqual(5)
+
+    // Aggregate return values remain the sum across all adjustments.
+    expect(Number(result.adjustmentsSubtotal)).toEqual(35)
+    expect(Number(result.adjustmentsTotal)).toEqual(35)
+  })
+
+  it("applies tax per adjustment when a taxRate is provided", function () {
+    const adjustments: any[] = [
+      { amount: 100, is_tax_inclusive: false },
+      { amount: 200, is_tax_inclusive: false },
+    ]
+
+    calculateAdjustmentTotal({ adjustments, taxRate: 0.1 })
+
+    // subtotal is the tax-exclusive amount; total includes 10% tax — per adjustment.
+    expect(Number(adjustments[0].subtotal)).toEqual(100)
+    expect(Number(adjustments[0].total)).toEqual(110)
+    expect(Number(adjustments[1].subtotal)).toEqual(200)
+    expect(Number(adjustments[1].total)).toEqual(220)
+  })
+})

packages/core/utils/src/totals/adjustment/index.ts

@@ -38,8 +38,11 @@ export function calculateAdjustmentTotal({
     adjustmentsTaxTotal = MathBN.add(adjustmentsTaxTotal, adjustmentTaxTotal)
     adjustmentsTotal = MathBN.add(adjustmentsTotal, adjustmentTotal)
 
-    adj["subtotal"] = new BigNumber(adjustmentsSubtotal)
-    adj["total"] = new BigNumber(adjustmentsTotal)
+    // Assign this adjustment's OWN subtotal/total — not the running cumulative
+    // sum across all adjustments. Using the plural accumulators here inflated
+    // every adjustment after the first to the cumulative total.
+    adj["subtotal"] = new BigNumber(adjustmentSubtotal)
+    adj["total"] = new BigNumber(adjustmentTotal)
   }
 
   const quantity = item?.quantity || MathBN.convert(1)