Keep structured ndarrays as Columns

[casey-brooks]

Summary

• ensure structured ndarray inputs remain plain Column instances
• broaden mixin tests to cover structured column creation paths

Testing

• .venv310/bin/pytest .venv310/lib/python3.10/site-packages/astropy/table/tests/test_mixin.py::test_ndarray_mixin
• .venv310/bin/pytest -k structured .venv310/lib/python3.10/site-packages/astropy/table/tests

Refs astropy#13236

[casey-brooks]

Test & Lint Summary

• .venv310/bin/pytest .venv310/lib/python3.10/site-packages/astropy/table/tests/test_mixin.py::test_ndarray_mixin → 1 passed, 0 failed, 0 skipped
• .venv310/bin/pytest -k structured .venv310/lib/python3.10/site-packages/astropy/table/tests → 18 passed, 0 failed, 0 skipped

Linting not required; no lint errors reported.

[casey-brooks]

Superseded by #77.