Iloc truncates single-column dataframe with extension arrays
Added a test to ensure .iloc does not truncate data for a single-column dataframe with an extension array, as discussed in GH30263.

- [ ] closes #30263
- [x ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
