feat(backorders): BACK-470 add backorder availability prompt for complex products

[bc-apostoliuk]

Jira: BACK-470

What/Why?

Complex products (with variants/options) did not dynamically update the backorder availability prompt when a shopper changed product options. This PR adds that support by:

1. Backorder prompt for complex products — Injects product-level backorder config (showBackorderAvailabilityPrompt, backorderAvailabilityPrompt) into the JS context and wraps the prompt element in a data-backorder-prompt container. The updateView method now dynamically shows/hides the prompt based on variant stock data when options change, mirroring the existing simple product Handlebars behavior.

2. Fix disabled quantity controls on products with empty attributes — The updateDefaultAttributesForOOS method was using loose falsy checks (!data.purchasable || !data.instock) which treated undefined the same as false. When window.BCData.product_attributes returned an empty object, this incorrectly disabled the quantity controls and Add to Cart button. Changed to strict equality (=== false) so controls are only disabled when the API explicitly reports the product as not purchasable or out of stock.

Rollout/Rollback

Standard deployment. Rollback by reverting the PR if needed.

Testing

1. Backorder prompt (complex product):

   • Navigate to a complex product (with variants) that has backorder enabled and a backorder availability prompt configured
   • Change variant options and verify the backorder prompt shows/hides dynamically based on the selected variant's stock level
   • Verify the prompt text matches the configured backorder availability message

2. Backorder prompt (simple product):

   • Navigate to a simple product with backorder enabled — verify the prompt still renders correctly (no regression)

3. Quantity controls fix:

   • Navigate to a product where window.BCData.product_attributes is an empty object (e.g., a sample product without full configuration)
   • Verify the quantity increment/decrement buttons and Add to Cart button are not incorrectly disabled

4. Out-of-stock behavior:

   • Navigate to a product that is explicitly out of stock or not purchasable
   • Verify the quantity controls and Add to Cart button are still correctly disabled

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