Create or update orders by external identifier

Create or update multiple orders by external identifier (if an order with a given external identifier exists it is updated, otherwise a new one is created).

Always returns HTTP 200 with one entry per input order, at the same index. Each entry is either the upserted order (carries id) or an error object {externalId, errorCode, message} when that single order failed — a failure on one order does not fail the batch, so inspect each entry (presence of errorCode = failure) rather than relying on the HTTP status. Soft validation issues still return as order entries with validationErrors populated and status=needs_attention. For a structured {results, summary} envelope, use POST /v2/orders/upsert/bulk.