Extracts the API call limit (used/total) from the response's x-shopify-shop-api-call-limit header.
x-shopify-shop-api-call-limit
The response object.
A tuple of [used, limit], or undefined if the header is missing/invalid.
Extracts the API call limit (used/total) from the response's
x-shopify-shop-api-call-limitheader.