Cart query fails when a line references a deleted or unpublished product

[hta218]

Description

When a cart line references a product that has been deleted or unpublished from the Hydrogen sales channel, the Storefront API returns null for the non-nullable field ProductVariant.product. That fails the entire CartQuery, not just the affected line.

In Hydrogen's `storefront.query`:
Cannot return null for non-nullable field ProductVariant.product
To debug the query `CartQuery`, try it in GraphiQL.
To investigate the issue, examine this stack trace:
  at loader (app/routes/api/cart.ts:21)

Two consequences:

• cart.get() throws in the loader, so /api/cart returns a 500. The cart bootstrap fails on every page, leaving the cart drawer stuck on its loading spinner site-wide — not only on the cart page.
• The cart cannot recover on its own. CART_MUTATION_FRAGMENT selects product as well, so add/remove/update mutations fail the same way. The cart id stays permanently unusable until the shopper is issued a new one.

Affected call sites of cart.get():

• app/routes/api/cart.ts
• app/routes/cart/cart-page.tsx
• app/routes/cart/checkout.tsx

Query fragments selecting merchandise { ... on ProductVariant { product { ... } } }: app/graphql/fragments.ts (CART_QUERY_FRAGMENT and CART_MUTATION_FRAGMENT).

Steps to reproduce

1. Add a product to the cart.
2. Delete the product, or unpublish it from the Hydrogen sales channel.
3. Reload any page on the storefront.

Notes

A defensive wrapper around cart.get() (catch, log, return null) was prototyped and reverted — it contains the crash but does not restore the poisoned cart. Recovering the cart needs a separate decision, since resetting the cart id on any query failure would also discard valid carts on transient API errors.