Migrate demoextrafield extra-field validation to Symfony constraints

[jolelievre]

Description

Migrates the demoextrafield example module from the legacy single validator: 'isXxx' string to Symfony Validator constraints (constraints: [...]), matching the core change in PrestaShop/PrestaShop#41806.

Each registerExtraProperty(...) definition now passes real Symfony\Component\Validator\Constraint objects:

• video_link (LANG) is the showcase for both multilang validation styles on one field:
  • new DefaultLanguage(allowNull: true, fieldName: 'Video link') — whole-array: if any language is filled, the default language must be too;
  • new Assert\All([new Assert\Url()]) — per-language: each language value must be a valid URL.
• custom_date (SHOP) → new Assert\Date().
• is_dangerous (COMMON, SwitchType) → no constraints — demonstrates that validation is opt-in (a switch always yields a valid 0/1).
• date_last_seen (COMMON) stays read-only, auto-updated by the FO hook.

hookDisplayFooterProduct is updated to load the product in a single language (new Product($productId, false, $languageId) — note langId is Product's 3rd ctor argument) and update date_last_seen (COMMON) plus, only when it's non-empty, the per-language video_link via ObjectModel::update() — exercising constraint validation in the front-office legacy container.

Depends on

• Replace legacy extra-property validation with Symfony Validator constraints PrestaShop#41806 — the core constraint-based validation (required for this module to validate).
• Replace displayApi flag with associatedApis in demoextrafield #249 — associatedApis migration; this branch is stacked on it and will be rebased onto master once Replace displayApi flag with associatedApis in demoextrafield #249 merges.

How to test

1. Check out the core branch from Replace legacy extra-property validation with Symfony Validator constraints PrestaShop#41806 on a 9.2 dev shop.
2. Install demoextrafield; open a product form.
3. Video link: invalid URL in any language → per-language URL error; fill a non-default language but leave the default empty → DefaultLanguage error; empty everywhere → passes; valid → persists.
4. View the product on the front office → footer hook updates date_last_seen and (when set) the current language's video_link.