Test column addition using .at with different values
- [x] closes #30649 
@jreback I am tagging you since you added a few labels to the PR a couple of days ago. Failing check seems not related to this PR. Is there something more that can be done here from my side at the moment or should I simply wait for the review? This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.