BUG: .describe() doesn't work for EAs #61707

[kernelism]

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.

• closes BUG: .describe() doesn't work for EAs #61707
• Adds a regression test.
• pre-commit checks passed
• Adds type annotations
• Adds a whatsnew entry