Fraud Protection: Add event dispatching for pay-for-order page

[leonardola]

Summary

Adds event tracking to CheckoutEventTracker for the pay-for-order flow (WOOSUBS-1387):

• pay_for_order_page_loaded event — dispatched via template_redirect when is_checkout_pay_page() is true. The regular checkout_page_loaded event is now excluded from pay-for-order pages to avoid duplicate/misleading signals.
• order_placed event on successful payment — hooks into woocommerce_before_pay_action to register a woocommerce_payment_complete listener scoped to the pay-for-order flow. This is needed because the regular checkout order_placed tracking (via woocommerce_checkout_order_processed) doesn't fire in the pay-for-order path.

Why this approach

The pay-for-order flow in WooCommerce (WC_Form_Handler::pay_action()) doesn't fire woocommerce_checkout_order_processed, so the existing checkout event tracking misses it entirely. We use woocommerce_before_pay_action as a trigger to conditionally register the woocommerce_payment_complete listener — this way the listener is only active during pay-for-order requests, not globally.

Test plan

Pay-for-order page load event

1. Create a WooCommerce order with status "pending payment" (e.g., place an order with "Check payments" gateway)
2. Navigate to the pay-for-order page: /checkout/order-pay/<order_id>/?pay_for_order=true&key=<order_key>
3. Verify in WooCommerce logs (woo-fraud-protection source) that a pay_for_order_page_loaded event is collected
4. Verify that checkout_page_loaded is not collected on this page

Regular checkout unaffected

1. Navigate to the regular checkout page
2. Verify checkout_page_loaded is still collected as before
3. Verify pay_for_order_page_loaded is not collected on regular checkout

Payment completion tracking

1. Go to the pay-for-order page for a pending order
2. Complete payment with a test gateway (e.g., Stripe test mode)
3. Verify an order_placed event is collected with the correct order details (order_id, payment_method, total, currency, customer_id, status)

Failed payment

1. Go to the pay-for-order page for a pending order
2. Attempt payment with a card that will be declined
3. Verify no order_placed event is collected (since woocommerce_payment_complete doesn't fire on failure)

🤖 Generated with Claude Code

[luizreis]

Let's drop the payment completion tracking (track_order_paid_via_pay_for_order, track_payment_complete, the woocommerce_before_pay_action hook, and their tests) from this PR.

• order_placed and payment completion are different events. In this case, the order was placed (created) before the user reached the pay-for-order page. However, the code on this PR tracks payment, not placement. If we need payment completion tracking later, it should be a distinct event name.
• The verify call happens before payment, so this event doesn't inform the blocking decision, and it would only appear in a future verify call's collected_events.
• It might not work for async/redirect gateways since payment_complete fires in a different request where the listener isn't registered.

IMO, the page load tracking with a proper event name is the valuable part and stands well on its own.

[luizreis]

LGTM! Thanks for addressing the comments