BUG: Fix dir(interval_index)
- [x] closes #27571
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

> I think the proper fix is to infer_dtype for letting it handle EAs correctly

Yah, the fix I'm working on locally makes infer_dtype handle interval correctly.  Getting all EAs right is out of scope. thanks @jbrockmendel 