Create dataset items from JSON file

Create dataset items from an uploaded JSON or JSONL file. JSON files must contain a top-level array of objects. JSONL files contain one JSON object per non-blank line; multi-line JSON objects are not supported. Reserved keys (id, source, description, tags, evaluators, execution_policy) are extracted into the corresponding DatasetItem fields; all remaining keys form the item's data map and preserve their JSON types. To link dataset items to specific traces or spans use the dedicated /items/from-traces or /items/from-spans endpoints. Processing happens asynchronously in batches. With dataset versioning enabled, a supplied id acts as an upsert key.