LLM schema: unknown products return an empty snippet, not 404

A wrong store or product ID doesn't return an error. Check for reviews before you render.

• Unknown IDs: the endpoint returns 200, not 404. An unknown product gets a Product with an empty name and a reviewCount of 0, and an unknown store gets an empty review block. The code samples now skip snippets without reviews.
• Accept header: send none, or */*. A request that accepts only text/html or application/json gets 406, which is now documented.
• Verification: look for application/ld+json or yotpo-reviews-section-data in the page source, depending on your account.

See Retrieve LLM schema HTML for a product and the Add review content for AI crawlers recipe.

Endpoints affected: Retrieve LLM schema HTML for a product.