Add NewCustomerResolverInterface extension point for new_customer field

[koenvdwetering]

Summary

The new_customer field in the data layer is currently hardcoded to $order->getCustomerIsGuest() in three separate call sites:

• DataLayer/Event/Purchase.php:84 — used by the MageWire/Hyva customer-data push via Observer/TriggerPurchaseDataLayerEvent
• DataLayer/Tag/Order/UserData.php:57 — used by the shared Luma/Hyva success-page layout (view/frontend/layout/checkout_onepage_success.xml)
• DataLayer/Event/PurchaseWebhookEvent.php:125 — used by the server-to-server webhook

Two problems with this:

1. No upgrade-safe extension point. A merchant plugging into one location silently misses the other two. There is no ProcessorInterface hook for the webhook path, and afterGet plugins on Purchase don't cover the success-page tag.
2. isGuest() is a poor proxy for "new customer". A returning customer can check out as a guest, and a new customer can register an account. More importantly, shops that convert guest orders into customer accounts on sales_order_place_after observe isGuest() === false by the time the value is read, so Google Ads' new-customer bid-modifier never receives a signal.

This PR extracts the check into NewCustomerResolverInterface (isNewCustomer(OrderInterface): bool) with a default IsGuestResolver implementation that preserves existing behavior. All three call sites now delegate to the resolver.

For merchants

Override the default by adding a preference in your own module's di.xml:

<preference for="Tagging\GTM\Api\NewCustomerResolverInterface"
            type="Vendor\Module\Model\PriorOrderCountResolver"/>

Backwards compatibility

No behavior change for existing installations — the default resolver returns (bool)$order->getCustomerIsGuest(), matching the previous three hardcoded expressions.

Test plan

• bin/magento setup:di:compile succeeds (new interface + preference resolve)
• Existing TriggerPurchaseDataLayerEventTest integration test still passes
• Guest checkout on success page emits new_customer: "true" in dataLayer (default behavior preserved)
• Registered customer checkout emits new_customer: "false" (default behavior preserved)
• Webhook payload contains matching new_customer value
• A custom preference (e.g. always-true resolver) takes effect across all three emit points

https://claude.ai/code/session_01KrUuLFtrNMyY2ZuwrvH8db

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