Use np.real and np.imag when possible

[Illviljan]

This reduces NamedArray's minimum requirements for a duckarray.

For example pd.Index doesn't have .real and .imag (or __array_function__/__array_namespace__ unfortunately, a problem for another PR...).

xref: #9314

[Illviljan]

================================== FAILURES ===================================
________________________ xarray/tests/test_backends.py ________________________
[gw0] win32 -- Python 3.12.5 C:\Users\runneradmin\micromamba\envs\xarray-tests\python.exe
worker 'gw0' crashed while running 'xarray/tests/test_backends.py::TestNetCDF4ViaDaskData::test_roundtrip_coordinates'

flaky?

[TomNicholas]

flaky?

Everything "dask roundtrip" seems to be somewhat flaky in my experience.

[max-sixty]

(I would be +1 on xfailing anything like that, not as good as fixing but better than letting it break some reasonably high % of CI...)