feat(product): formalize product data contract and decouple from assessments

[FAMarfuaty]

Context

Part of the API-migration work to make the content-analysis engine consumable off-WordPress. The two e-commerce SEO assessments (Product identifiers, SKU) were coupled to WooCommerce's productType enum and read their input from an implicit, undocumented customData object. This formalizes that input as a typed, validated productData contract and replaces the platform-specific enum with a neutral isVariableProduct boolean, so non-WordPress/headless consumers can reproduce in-editor scores.

Stacked on #23336 (the PaperDTO input contract, #634); this should merge after it lands.

Summary

This PR can be summarized in the following changelog entry:

• [yoastseo 0.1.0 enhancement] Decouples the e-commerce Product identifiers and SKU assessments from the WooCommerce-specific product-type enum.

Relevant technical choices:

• productData is a new first-class Paper field with a zod schema; the existing customData stays as a back-compat fallback so unmigrated producers (and npm consumers) keep working without a coordinated release.
• The productType → isVariableProduct derivation lives in a consumption-boundary narrower (normalizeProductData), not a zod .transform(), because the in-editor path builds the Paper directly and never crosses the contract boundary — so the defaulting has to happen where the assessment reads the data.
• That narrower is kept in a zod-free module and imported directly by the assessments (not via the contract barrel), so zod stays out of the core analysis bundle — measured ~60 KB smaller versus importing through the barrel.
• Collapsing the duplicated productType string-lists to a single boolean also removed a latent applicability asymmetry in the SKU assessment (it omitted grouped); verified inert for real WooCommerce data and pinned with a test.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This is engine plumbing with no intended change for the standard product types. It is covered by unit tests (yarn workspace yoastseo test, contract + assessment specs). For behavioural parity:

Note

Test together with this https://github.com/Yoast/wpseo-woocommerce/pull/1164

1. Edit WooCommerce products of each type (simple, variable, grouped, external), with and without a global SKU and identifiers, and — for variable products — with and without all variants carrying SKUs/identifiers.
2. Confirm the Product identifiers and SKU assessments show the same scores as on trunk (the back-compat customData fallback keeps unmigrated producers working even without the matching wpseo-woocommerce PR #1164).

Test instructions for the Shopify app

Shopify is not modified by this PR — it still sends its product data through the legacy customData bag — so this is a regression check that the back-compat fallback keeps it working:

1. In the Shopify app, open a product and its SEO analysis.
2. With and without a SKU, and with and without a barcode (product identifier), confirm the SKU and Barcode (Product identifiers) assessments show the same results as before this change.
3. For a product with multiple variants, confirm the assessments behave as before (Shopify is assessed as a single unit, not per-variant).
4. Keep the browser console open and confirm there are no errors.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• The Product identifiers and SKU assessments, the Paper value object (new productData attribute), and the PaperDTO input contract. WooCommerce and Shopify are the only producers of this data; both keep working through the customData back-compat fallback.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have run grunt build:images and committed the results, if my PR introduces or edits images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Part of Yoast/lingo-other-tasks#657

[github-actions]

A merge conflict has been detected for the proposed code changes in this PR. Please resolve the conflict by either rebasing the PR or merging in changes from the base branch.