Build a Shopify Function for product discounts using Rust or JavaScript with a run.graphql input query

Steps

1. Scaffold a new Shopify app with the CLI and add a product discount Function extension using `shopify app generate extension`, selecting the product discount Function type.
2. Define your `run.graphql` input query to declare exactly which cart and product fields your Function needs — Shopify only provides fields you explicitly request in this query.
3. Implement the `run` entrypoint (in Rust: via the `shopify_function` macro; in JS: as the default export) to receive the typed input and return a `FunctionRunResult` with a `discounts` array specifying targets and percentage or fixed-amount values.
4. Use `shopify app dev` to run the Function locally against a development store and trigger it by adding qualifying products to a cart, inspecting the discount result via the Function tracing panel in the Partner Dashboard.
5. Register the Function as a discount automation via the Admin GraphQL API (verify mutation name against current API docs, e.g. a variant of `discountAutomaticAppCreate`) with a title, function ID from the extension handle, and `combinesWith` settings.
6. Test edge cases: zero-item carts, products not in scope, combine-with-other-discounts behavior, and maximum discount caps enforced by Shopify.