Read the results of a task, a page at a time

The results of an enrich_persons task in the order sent, one TaskResultLine per item, as in the NDJSON file, readable from the start of the run. While the task runs, a page stops at the first item not finished: items is empty, with Retry-After, while the next one runs, and a cursor never skips an item. A cancelled task still finishes the items it had started, within the hour: a page waits for them the same way. Pass nextCursor back as cursor; it is null once nothing more will come. A page can hold fewer items than limit (at most 100, 2 MB of JSON). A cursor keeps working once the task settles. A task stored before reading by page existed: 409 TASK_RESULTS_FORMAT_UNAVAILABLE, its CSV stays available at /v2/tasks/{taskId}/results. Too many pages read at once: 429 TASK_ROWS_BUSY, retry after Retry-After. Credits: 0 (free).