Try the Costco search API

- Each result's `id` is usable directly as the `id` path parameter to [`/costco/product/{id}`](costco-product.md). - A missing `query` and `category` returns `400` before any upstream request is made. - When the GDX search service is blocked, query-only requests may use a partial fallback. In that response, `source` is `product_sitemap` and `is_partial` is `true`. The `total_count` then counts product URL slugs matching the query, not Costco's GDX result count, and the order follows Costco's product sitemap rather than GDX relevance. Other source values are not currently returned; `source` is omitted for complete GDX results. - The product-sitemap fallback does not handle category filters and does not return success when no product slug matches the query. - If GDX is blocked, text-only queries may fall back to actual products from Costco's public product sitemap, enriched from its product GraphQL API. Those responses include `source: "product_sitemap"` and `is_partial: true`. In that case `total_count` counts matching sitemap URL slugs, the product order follows the sitemap, and results may be fewer than Costco's GDX result set. This fallback is not used for category-filtered queries or when no sitemap product matches; those calls continue returning an upstream error.