Write a Shopify Function for cart and checkout validation that blocks checkout if a custom business rule is violated

Steps

1. Generate a cart and checkout validation Function extension via the Shopify CLI.
2. In `run.graphql`, declare the cart fields needed for your rule — line items, quantities, product metafields, buyer identity — keeping the query minimal to stay within the instruction budget.
3. In the Function `run` body, evaluate the rule (e.g., maximum quantity per SKU, restricted product combinations) and return a `FunctionRunResult` with a `errors` array when the rule is violated; each error requires a `localizedMessage` string and a target.
4. When the rule passes, return an empty `errors` array — the Function must always return a valid result, not throw.
5. Test on a dev store by adding products that trigger the validation; the checkout UI will display the localized error message and block the buyer from proceeding.
6. Use the Partner Dashboard Function traces to inspect the input payload and returned errors for debugging.