Clarified how to specify sale price effective dates in Product structured data

What : Added a new section on “Sale duration” to the Merchant listing guide . This explains how to use the validFrom , validThrough , and priceValidUntil schema.org properties to set the effective range for sale prices, including best practices and examples for placement on either Offer or PriceSpecification nodes. Why : This aligns schema.org usage with the Merchant Center feed attribute sale_price_effective_date , providing clear instructions and best practices for merchants using structured data.