Adds metabox on the Subscription edit screen when HPOS is enabled

[worais]

All Submissions:

• Does your code follow the Extendables standards?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changes proposed in this Pull Request:

Adds support for displaying the Amazon Pay metabox on the Subscription edit screen when HPOS is enabled.

Previously, the metabox only appeared on classic order edit screens (shop_order).
When HPOS was activated, the screen ID for subscriptions changed (woocommerce_page_wc-orders--shop_subscription), causing the metabox not to render.

This PR:

• Adds a new utility method WC_Amazon_Payments_Advanced_Utils::get_edit_subscription_screen_id().
• Updates the admin class WC_Amazon_Payments_Advanced_Order_Admin to:
  • Register the metabox for both Orders and Subscriptions in classic and HPOS modes.
  • Handle non-AJAX order actions for subscriptions as well.

Closes woocommerce#296.

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How to test the changes in this Pull Request:

1. Activate WooCommerce Subscriptions and Amazon Pay.
2. Enable HPOS in WooCommerce → Settings → Advanced → Features.
3. Create a Subscription paid with Amazon Pay.
4. Open the Subscription edit screen in the admin.
   • ✅ Expected: the Amazon Pay metabox is visible and functional.
5. Disable HPOS and confirm that the metabox still appears normally.

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Other information:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

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Changelog entry

Fix – Display Amazon Pay metabox on Subscription edit screen when HPOS is enabled.

[alexiglesias31]

LGTM!