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Changes proposed in this Pull Request:

A modal checkout link of the form ?checkout=1&type=checkout_button&product_id=NNN&variation_id=MMM now opens the checkout modal for the requested variation. Previously these links did nothing for variable subscription and grouped products: no modal appeared, and depending on the product the URL parameters were either stripped from the address bar or left stuck there.

The trigger now resolves the correct target in every configuration:

• A checkout button locked to the requested variation opens that variation directly.
• A variation different from the button's locked one is opened through the variation picker, so an arbitrary requested variation is honored.
• Grouped product children are opened through the picker as well.
• Variable subscription products now render their picker even when a button is locked to a single variation, so any of their variations can be requested by URL.

When a link requests a variation that cannot be matched to any product or picker, the checkout is no longer silently swapped for a different product. Nothing is submitted, the parameters stay in the URL, and a warning is logged so the broken link can be diagnosed. The URL-driven picker checkout also carries the source button's context (after-success behavior, content gating, popup attribution), matching a manual click. Normal clicks on checkout buttons are unchanged.

Closes NPPM-2872.

How to test the changes in this Pull Request:

These steps assume a site with Reader Activation and modal checkout enabled, WooCommerce and WooCommerce Subscriptions active, a variable subscription product with several variations, a grouped product, and a simple subscription product. Use a page with Checkout Button blocks: one linked to the variable subscription product locked to a specific variation, one linked to the grouped product, and one linked to the simple subscription product. Test logged out (or in a private window) and keep the browser console open throughout.

1. Visit PAGE?checkout=1&type=checkout_button&product_id=VARIABLE_PARENT_ID (no variation_id). Confirm the modal opens for the button's locked variation and the URL parameters are cleared.
2. Visit PAGE?checkout=1&type=checkout_button&product_id=VARIABLE_PARENT_ID&variation_id=LOCKED_VARIATION_ID. Confirm the modal opens for that variation.
3. Visit the same URL but with variation_id set to a different variation of the same product (not the locked one). Confirm the modal opens for that other variation, and that the summary shows the requested variation rather than the locked one.
4. Visit PAGE?checkout=1&type=checkout_button&product_id=GROUPED_ID&variation_id=CHILD_PRODUCT_ID. Confirm the modal opens for that child product.
5. Visit a checkout button URL whose variation_id does not belong to the product, for example product_id=SIMPLE_SUBSCRIPTION_ID&variation_id=UNRELATED_ID. Confirm no modal opens, the URL parameters remain in the address bar, and a Newspack modal checkout: no checkout form found... warning is logged.
6. Throughout steps 1 to 5, confirm there is no Uncaught SyntaxError: "undefined" is not valid JSON in the console.
7. Confirm normal behavior is unchanged: clicking a locked checkout button goes straight to checkout, and clicking a grouped or chooser button opens the variation picker.
8. Confirm an after-success setting on the source button (for example a custom button label or redirect) is honored when the checkout is opened through a URL-driven picker variation.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?