Core SDK loading & PayPal buttons via v6 Web SDK (5780)

[danieldudzic]

Note: This PR is against the v6 SDK epic branch: dev/PCP-5777-update-v-5-js-sdk-integration-to-v-6-web-sdk and not dev/develop

Description

Core v6 Web SDK frontend: createInstance() with a server-fetched client token, explicit eligibility via findEligibleMethods() (currency/country/amount), one-time payment sessions and Web Component buttons for PayPal/Venmo/Pay Later across product, cart, checkout, and classic mini-cart.

• Server side is 100% shared with v5. v6 calls the existing ppc-* endpoints through one JS adapter and reuses existing services/helpers (SettingsStatus, styling DTOs, Context, SingleProductActionHandler, ErrorHandler). Only change outside the module: one do_action in CreateOrderEndpoint (stores the session order for non-checkout contexts; v5 unchanged).
• Completion flow matches classic v5 continuation: approve, redirect to checkout, Place Order processes the session order.

Migration methodology (v5 and v6 side by side): during the epic, every payment surface is owned by exactly one stack. A single function (SdkV6Manager::should_load_on_current_page()) decides per page whether v6 loads. On those pages the v5 smart buttons are suppressed, since both SDKs claim window.paypal. Everywhere else (block cart/checkout, pay-now, wallets, messaging) v5 keeps running untouched. This keeps the site and the QA/e2e suites fully functional after every story lands: each subsequent ticket simply widens what v6 owns instead of adding per-surface toggles or shims. The handoff is migration-phase scaffolding only, marked for removal at release (see extensions.php). The end state is a whole-flow flag flip: v5 or v6, never a per-page mix.

Screenshots

Before	After

Missing on the v6 side by design: messaging, wallets - included in separate tickets

Steps to Test

Setup: flag on via PCP_SDK_V6_ENABLED=1 (or filter woocommerce.feature-flags.woocommerce_paypal_payments.sdk_v6_enabled set to true), sandbox-connected store, npm run build.

1. Product page: gold PayPal/Venmo/Pay Later Web Components render in #ppc-button-ppcp-gateway-v6. Network shows web-sdk/v6/core plus one ppc-sdk-v6-client-token POST, and no paypal.com/sdk/js. No [PPCP SDK v6] console errors.
2. Purchase: click PayPal, approve as sandbox buyer, get redirected to checkout, click Place Order. Verify the order in wp-admin (Processing, PayPal txn ID, stock reduced).
3. Shipping in popup: from cart, change the address to another US state. update-customer (non-empty state) and ppc-update-shipping fire, popup totals update.
4. Settings: shape/color changes in Styling apply; disabling a the script there.
5. Coexistence / flag off: block cart/checkout still show v5 e off, no v6 anywhere (identical to dev/develop).
6. Suites: npm run unit-tests, npm run test:unit-js, npm cp-blocks/checkout-handler JS failures are unrelated; they fail on
   dev/develop too).

[github-actions]

Test using WordPress Playground

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What's included:

• ✅ WordPress (latest)
• ✅ WooCommerce (latest)
• ✅ PayPal Payments plugin v4.1.1-pr4526-29766602071-g2446251 (built from this PR)

Login credentials:

• Username: admin
• Password: password

Plugin Details:

• Version: 4.1.1-pr4526-29766602071-g2446251
• Commit: 17f8fac
• Artifact: woocommerce-paypal-payments-4.1.1-pr4526-29766602071-g2446251

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[stracker-phil]

Ready to merge; all comments are nits/suggestions

[Narek13]

Really nice PR, thank you! It looks great to me. I’ve also added a couple of small suggestions on top of what @stracker-phil already mentioned, but I think we’re good to go. Thanks again! 🙂