Notify Me on preorder products can appear for variants not included in a STOQ Preorder offer

We detected a gap in the current Show Notify Me button on preorder products behavior.

At the moment, STOQ determines whether a variant is a preorder for this setting using only the Shopify inventory state:

inventory_policy = continue

inventory_quantity <= 0

It does not verify whether the variant is actually included in an active STOQ Preorder offer/selling plan.

Because of this, any variant using Shopify’s Continue selling when out of stock option with zero or negative inventory can show the Notify Me button when show_button_on_preorder is enabled, even if that variant has never been configured as a preorder in STOQ.

Requested improvement: When Show Notify Me button on preorder products is enabled, also verify that the selected variant belongs to an active/eligible STOQ Preorder offer before treating it as a preorder product.

This would prevent normal oversold/backordered variants from being treated as STOQ preorder variants for Notify Me visibility.