Adds one or more line items to a section on a draft quote. Line,
section, and quote-level totals are recomputed and persisted as
part of the call. New line items appear at the end of the section in
the quote editor.

Item-field validation reports all failing rules across the batch
in one 422 response, with paths such as line_items[i].field.

Response body matches GET /v1/quotes/{quote_id}.