TST: assert that informative error is raised when offset not supported as a period frequency is passed to DataFrame.asfreq
- [x] closes #56718 (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature (python -m pytest pandas/tests/frame/methods/test_asfreq.py)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] N/A: Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] N/A: Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@phofl anything I should do now to move this forward? this has been addressed by #56945

really sorry that you spent time on this one, and I hope you got something out of the experience anyway - i hate to do this, but i'll close then reopening, as:
- the tests from this PR are different to those in the linked one, so they would still be good to add
- I just didn't handle this well, I wasn't aware of this PR when I'd started reviewing the other one

will add a commit soon-ish (early next week) to just keep the tests, then we can merge it in

I didn't get this right as reviewer, I need to own up to this