Fix the `OptimisticCart` type to properly retain the generic of line items

[blittle]

WHY are these changes introduced?

@brookslybrand reported an issue where the the generic on OptimisticCart did not propagate to line items. For example, this would produce a type of the default cart lines, not the generic api fragment provided.

type CartLine = OptimisticCart<CartApiQueryFragment>['lines']['nodes'];

With this change, now the CartAPIQueryFragment is preserved.

Checklist

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

[shopify[bot]]

Oxygen deployed a preview of your bl-fix-optimistic-cart-types-2 branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:49 PM
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:49 PM
optimistic-cart-ui	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:49 PM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:49 PM
classic-remix	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:49 PM
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	July 29, 2024 6:50 PM

Learn more about Hydrogen's GitHub integration.

[blittle]

/snapit

[github-actions[bot]]

🫰✨ Thanks @blittle! Your snapshot has been published to npm.

Test the snapshot by updating your package.json with the newly published version:

"@shopify/hydrogen": "0.0.0-snapshot-20240712202726"

[merrickread]

I've experienced this as well! Would love to see it come in the next release :)