API: Include product_type in quote line item API response

Description:

When retrieving a quote via the API, the line item payload does not include the product_type field. To obtain this value, we currently have to make a separate call to /public.api/product/{id} for each line item. For quotes with many products, this results in a significant number of additional API calls.

Requested behaviour:

Include product_type directly in the quote line item response object, so callers can access it without a secondary lookup.

Why this matters:

product_type is a commonly needed field when processing or rendering quote data. Requiring an extra per-item call to retrieve it creates unnecessary overhead and degrades API ergonomics, particularly for integrations handling large quotes. A common alternative seen in well-designed APIs is either embedding the field by default or offering an ?expand= parameter to opt into enriched responses.

Current workaround:

Call /public.api/product/{id} for each line item to retrieve product_type separately.