BUG: item_cache invalidation on DataFrame.insert
- [x] closes #26985
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Something wonky is going on with tm.assert_produces_warning, will see if it shows up on the CI.