BUG: .describe() doesn't work for EAs #61707
This PR fixes a bug where Series.describe() fails on certain `ExtensionArray` dtypes such as `pint[kg]`, due to attempting to cast the result to `Float64Dtype`. This is because some of the produced statistics are not castable to float, which raises errors like DimensionalityError.

We now avoid forcing a Float64Dtype return dtype when the EA’s scalar values cannot be safely cast. Instead:

If the EA produces outputs with mixed dtypes, the result is returned with `dtype=None`.

- [x] closes #61707 
- [x] Adds a regression test.
- [x] pre-commit checks passed
- [x] Adds type annotations
- [x] Adds a whatsnew entry

@jbrockmendel This is a new PR with the comments from [61760](https://github.com/pandas-dev/pandas/pull/61760) addressed. Apologies for the duplication.  

Looks like this breaks some other tests