Pollyfill getSetCookie if not available.

[blittle]

WHY are these changes introduced?

With the upgrade to React Router 7, we are no longer polyfilling fetch. The local preview of Hydrogen runs in an older version of workerd, which does not have getSetCookie implemented. The real solution here is to upgrade to the latest version of miniflare under the hood. This fixes it in the meantime.

Fixes #2959

HOW to test your changes?

1. Run npm install and npm run build from the root.
2. Run npm run preview from the skeleton template
3. Add an item to cart.

Post-merge steps

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[shopify]

Oxygen deployed a preview of your bl-2959 branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	May 23, 2025 3:19 PM
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	May 23, 2025 3:19 PM
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	May 23, 2025 3:19 PM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	May 23, 2025 3:19 PM

Learn more about Hydrogen's GitHub integration.

[github-actions]

This pull request has been marked as stale due to inactivity for 60 days. If no further activity occurs, it will be closed in 7 days.