BUG: groupby.agg returns incorrect results for uint64 cols (#26310)
- [x] closes #26310
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A way to avoid incorrect coercion to float64 in the case of uint64 input is to introduce an additional check to `ensure_int64_or_float64`
