Cart subtotal ignores item quantity

Reproduction

Run:

pnpm install
pnpm test:pricing

Expected

calculateCartSubtotal should multiply each line item's unitPrice by quantity. The sample cart should total 3150 cents.

Actual

The test reports that the subtotal is 1450 cents instead of 3150 cents.

Notes

This is an intentional repro issue for testing a sandboxed HarnessAgent triage workflow.