Investigate/Fix paged query Error

#29
subject group = stat*
Dates = [202006120000+TO+202006170000
items per page = 500
total = 523

Fulfillment Item shows total result as 500 AND fetch span is negative

- [x] Verify if last 23 publications are missing
Samples taken show publications are present, this is due to the fact that many of these publications have already been persisted via other http request/fulfillment item rounds.
It is still quite possible to lost publications if the paged request error is not fixed.

- [x] Fix fetch span

Look at the paged query logic in the fulfillment item service, likely in a file handling arXiv scraping or data fetching. Check how total results and fetch span are calculated when items per page is 500 and total is 523. Run tests for paged queries to reproduce the negative fetch span and verify missing publications.

Written by the indexing model from the issue text.