fix: propagate SkyCoord descriptor AttributeError (swev-id: astropy__astropy-14096)

[casey-brooks]

Summary

• ensure SkyCoord.__getattr__ performs a static descriptor check before falling back to dynamic frame logic so subclass properties surface their own AttributeError
• add regression coverage for subclass descriptors, transform alias access, and frame attribute delegation

Reproduction / Observed Failure

import astropy.coordinates as coord

class custom_coord(coord.SkyCoord):
    @property
    def prop(self):
        return self.random_attr

c = custom_coord('00h42m30s', '+41d12m00s', frame='icrs')
c.prop

Observed prior to this change:

AttributeError: 'custom_coord' object has no attribute 'prop'

Verification Notes

• transform alias access (e.g., SkyCoord.icrs) continues to round-trip
• frame attribute delegation (e.g., SkyCoord.obstime) continues to expose frame metadata

Resolves #116.

[casey-brooks]

Test & Lint Summary

• LIBGCC_DIR=$(dirname $(gcc --print-file-name=libgcc_s.so)) ZLIB_DIR=/nix/store/4wdz42ns29ys6fm1xak68bnp51nxhd2s-zlib-1.3.1/lib LD_LIBRARY_PATH=$LIBGCC_DIR:$ZLIB_DIR SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. .venv/bin/pytest astropy/coordinates/tests/test_sky_coord.py -k 'subclass_property_attribute_error_propagates or transform_alias_access_unchanged or frame_attribute_delegation_unchanged'
  • 3 passed, 431 deselected
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. .venv/bin/flake8 --extend-ignore=E501,E203 --per-file-ignores='astropy/coordinates/tests/test_sky_coord.py:F841,E731' astropy/coordinates/sky_coordinate.py astropy/coordinates/tests/test_sky_coord.py
  • no lint findings (ignored E501/E203 and legacy F/E rules already waived in this file)

[noa-lucent]

Fix correctly short-circuits SkyCoord.__getattr__ when a descriptor is defined on the class so subclass properties raise their own errors again. Regression tests hit the descriptor case plus the alias/frame delegation paths, so coverage looks good. Approving.