Fix persistent cart guest access after APSB26-73

[srdyilmaz197-a11y]

Description (*)

APSB26-73 added guest-cart ownership validation that rejects a masked cart while its quote still has a customer ID. Magento_Persistent intentionally creates that state for remembered shoppers whose normal customer session has expired: the quote remains customer-owned while checkout is routed through guest-cart APIs.

This change converts only the active persistent customer's own quote at the shared masked-cart ID resolution boundary, before guest-cart services apply the new check. The conversion requires the persistent-session customer ID to match the quote owner ID, so a masked ID for another customer's quote remains rejected.

The conversion also marks the quote as guest, clears the emulated customer session, and removes the persistent cookie/session after the ownership match.

The plugin is registered for frontend, REST, and SOAP because Magento_Persistent participates in those guest checkout paths.

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes APSB26-73 guest cart check rejects persistent carts, breaking checkout when Magento_Persistent is enabled #41090

Manual testing scenarios (*)

1. Enable Persistent Shopping Cart, including Persist Shopping Cart, with Clear Persistence on Sign Out disabled.
2. Log in as a registered customer with Remember Me enabled and add a product to the cart.
3. Expire/delete only the normal PHP session cookie while keeping persistent_shopping_cart.
4. Reload checkout and confirm the remembered cart is restored while the shopper is not normally logged in.
5. Continue through guest-cart shipping/totals/payment requests using the masked cart ID.
6. Verify the persistent shopper's own quote is converted to a guest quote before guest-cart ownership validation and checkout can proceed.
7. As a negative case, use a persistent session whose customer ID does not match the masked quote owner and verify the quote is not converted.

Questions or comments

Unit and integration regression coverage is included. A full Magento/PHP test run was not available in the local laptop environment, so this is being opened as a draft for repository CI/maintainer validation.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

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