Fix “Missing value [age_group]” on Google Merchant Center

Apparel and accessories require age_group on every item. The values map to body-size standards: newborn, infant, toddler, kids, adult. Missing age_group blocks listing in apparel categories because channels can't apply gender-and-age-appropriate sizing logic.

The default-to-adult instinct works for adult apparel but mis-handles mixed catalogs. A catalog selling both women's clothes (adult) and kids' versions (kids) needs per-item age_group, not a universal default. Pipelines that default all items to adult put kids' apparel into the wrong competitive set.

For non-apparel categories where age_group is genuinely irrelevant (electronics, home goods, books), the field is optional. Populating it where there's a clear target audience (kids' toys, babies' products) helps targeting; populating universally adds noise.