ENH: Categorical.fillna allow Categorical/ndarray
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

xref #32414.

cc @TomAugspurger ATM the new test implemented in tests.arrays.categorical.test_missing is failing on the ndarray case for reasons that I dont fully grok.  Are you familiar with this?
I think the failing test case is consistent with accidentally using a code for a value? Perhaps test with a Categorical and ndarray containing something other than integers to see if that's the case. Looks like the problem is that ATM Categorical.fillna is altering in-place for Series `other` gentle ping; this is a blocker for tightening types (and avoiding unwanted code patterns) in Block.putmask and Block.where thanks, i think we need a whatsnew note as well, pls do in a followon