fix: prevent unhandled errors from superseded cart mutations

[fredericoo]

TL;DR: Rapid cart updates deliberately abort superseded requests, but the correlated Standard Event promise was left unobserved and surfaced those cancellations as console errors.

Before

Repeatedly updating the same cart line produced an unhandled AbortError for every superseded request, even though the cart handled the cancellation correctly.

After

Hydrogen observes the redundant correlated event promise while keeping the caller-facing action promise authoritative. Expected cancellations stay silent; genuine mutation failures still follow the existing error path.

What this changes

• Observes the deduplicated Standard Event promise before discarding it.
• Adds regression coverage for asynchronous correlated events cancelled by a newer cart mutation.

Developer impact

Includes a patch changeset for @shopify/hydrogen. There are no public API or runtime contract changes.

Out of scope

• Server-side cart conflict responses remain genuine failures and are not swallowed.

Risk

• Low. The change only handles a redundant promise after its event has already been correlated with the authoritative action promise.

How to Test

1. Run pnpm --filter @shopify/hydrogen build.
2. Run pnpm --filter @shopify/hydrogen-template-react-router dev.
3. Open http://localhost:5173, add a product, and open the cart drawer.
4. Click the line item's increase button repeatedly in quick succession.
5. Confirm the cart settles on the latest quantity without unhandled AbortError messages in the browser console.