Add getVariantsByIds: variant-level presentment prices

[qeepcologne]

Adds ShopifyStore.getVariantsByIds — a variant-level lookup that returns presentment prices.

Why

There is currently no way to ask this package for the price of a known set of variants in a given market.

• getProductsByIds looks like the right call, but its query matches ... on Product. nodes(ids:) is polymorphic, so passing variant gids returns empty entries — silently, not as an error.
• The product queries can't stand in either. getProductsQuery nests products(first: 250) → variants(first: 250) → sellingPlanAllocations(first: 250), plus collections(250), images(250) and media(250). Against a real catalogue that is orders of magnitude past the Storefront API's 1000-point query-cost limit, and it comes back as a bare Internal error. Looks like something went wrong on our end. with a request id rather than a cost message.

What

get_variants_by_ids.dart selects no nested connections, so the query costs roughly one point per id. It runs under @inContext(country: $country) like the other queries and is fed ShopifyLocalization.countryCode, so the prices are the presentment prices for the selected market. Ids that don't resolve to a variant are skipped.

The selected fields are exactly what ProductVariant.fromJson needs, so the method returns the package's own model rather than a bespoke shape.

Note on stacking

This branch sits on top of #150 and #151, so the diff here includes both. Happy to rebase onto whatever lands first, or to split it if you'd rather take this one on its own.

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