Add Product suppliers endpoints (associate, default, remove all)

[PrestaEdit]

Questions	Answers
Description?	Add the Product suppliers endpoints (associate, default, remove all)
Type?	new feature
BC breaks?	no
Deprecations?	no
Fixed ticket?	Related to PrestaShop/PrestaShop#39630
Sponsor company

Adds the Admin API endpoints to manage a product's supplier associations:

• PUT /products/{productId}/suppliers — SetSuppliersCommand (associate suppliers by id)
• DELETE /products/{productId}/suppliers — RemoveAllAssociatedProductSuppliersCommand
• PUT /products/{productId}/default-suppliers — SetProductDefaultSupplierCommand

These manage the associations only (per-supplier reference/price content is handled by a separate
command and can follow up). All return 204.

The integration test associates a fixture supplier, sets it as the default (verified via
product.id_supplier) and removes all (verified via the product_supplier table). Reuses the
product_write scope.

[jolelievre]

Closing in favor of #410, which centralizes all the pending Product-domain endpoints into a single PR: mutualizing the changes makes the review simpler, and it allowed rebuilding the integration tests so that every fixture is created through the API alone (with whole-structure assertions). @PrestaEdit your work is kept and you are co-authored on the commits of #410 — thanks!

Note: in #410 the three operations live in the consolidated ProductSuppliers resource together with the GET and the details PATCH, every write returns the updated supplier options, and the default supplier URI is singularized to /products/{productId}/default-supplier.