List product modifiers

## Get product modifiers Returns the full list of modifiers linked to the given product, each including its options. ### Authentication Requires a Merchant API key in `x-api-key` with scope `Merchant.product.read`. ### Path params - `productId` (required) — the product UUID. ### Behavior - Validates the product exists in your store (400 `Product with id <productId> not found` otherwise). - Returns an array of modifier objects (each with nested `options`). --- **Notas:** Returns a bare array (no envelope). Related endpoint: GET /v1/products/:id already embeds the same modifiers. Product must belong to the caller's store.