feat(backorders): BACK-471 display backordered quantity message on PDP

[bc-apostoliuk]

Jira: BACK-471

What/Why?

Screen.Recording.2026-02-19.at.14.22.15.mov

Display a dynamic "X will be backordered" message on the product detail page (PDP) when the selected quantity exceeds available on-hand stock.

• Inject availableOnHand and availableForBackorder product context into the template
• Add a hidden <p data-product-backordered> element in the stock form field area
• Add updateQtyBackorderedMessage(qty) method to ProductDetailsBase that calculates how many items will be backordered and shows/hides the message accordingly
• Call the update method on both quantity increment/decrement and manual keyup input

Rollout/Rollback

Standard deployment. Rollback by reverting the PR if needed.

Testing

1. Navigate to a product that has backorder availability configured (on-hand stock < max purchasable, with backorder enabled)
2. Increase the quantity beyond the on-hand stock level
3. Verify the "X will be backordered" message appears below the stock label
4. Decrease quantity back to within on-hand stock and verify the message hides
5. Manually type a quantity in the input field and verify the message updates on keyup
6. Verify no message appears for products without backorder availability

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