Remove Skeleton PDP first variant redirect

[scottdixon]

WHY are these changes introduced?

Resolves https://github.com/Shopify/hydrogen-internal/issues/120

At the moment the Skeleton PDP 302 redirects to the first available variant. This was done to make the code simpler (the loader exclusively relies on search params). But this negatively affects performance

WHAT is this pull request doing?

This update removes the redirect by leveraging the Storefront API's new product.selectedOrFirstAvailableVariant field.

It also updates the VariantSelector to accept a new (optional) selectedVariant prop for default selected values. Since no URL params are set on the initial page load, the variant selector uses these values to highlight selected values.

A canonical link has also been added to the product page to avoid SEO issues with multiple URLs pointing to the same place.

HOW to test your changes?

Run the Skeleton PDP locally against a variety of products. Test products with varying options and availability.

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 sd-remove-pdp-redirect branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	November 14, 202412:26 AM
classic-remix	✅ Successful (Logs)	Preview deployment	Inspect deployment	November 14, 202412:26 AM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	November 14, 202412:26 AM
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	November 14, 202412:26 AM
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	November 14, 202412:26 AM

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[wizardlyhel]

Minor changes