GeezerVison

Moves request body decoding and per-price validation out of the site-wide upsertAIModelPrices handler, so the organization price handler later in this stack can reuse them.

• The handler still reads the body and writes the HTTP errors. writeAIModelPricesBodyReadError picks 413 or 400.
• decodeAIModelPricesBody only decodes, and returns an error.
• aiModelPriceKeys holds the JSON keys present in one price, so validation can tell an explicit null apart from a missing price.
• validateAIModelPriceValues checks the four prices of one model price. It takes them as an aiModelTokenPrices struct, so callers set each price by name and cannot pass them in the wrong order.

No behavior change. The existing TestUpsertAIModelPrices, TestListAIModelPrices and TestValidateAIModelPrices cover the site-wide handler.

Used later: #30720 reuses the body read error, the price keys and the value validation for its single-price upsert.

This PR was generated by Coder Agents on behalf of @SasSwart.