Refactor image tests

[smcastro]

Following the refactoring of test_image.py from #536 and following the approach used in #506 , we need to complement the work by moving the helper functions used in test_image (and now duplicated into test_image_xds) to the testing module so that they can be imported in xradio's tests and externally from other repositories.

[smcastro]

Refactor test_image.py and create xradio.testing.image

Files touched

File	Status
src/xradio/testing/image/__init__.py	Created
src/xradio/testing/image/io.py	Created
src/xradio/testing/image/generators.py	Created
src/xradio/testing/image/assertions.py	Created
tests/unit/image/conftest.py	Created
tests/unit/image/test_image.py	Modified (854 lines, was 1081)
src/xradio/testing/__init__.py	Modified

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1 — New subpackage: src/xradio/testing/image/

Mirrors the existing xradio.testing.measurement_set layout. All functions are framework-agnostic (no pytest dependency) so they are usable from ASV benchmarks and third-party projects.

io.py

New function	Signature	Extracted from (old file)	Purpose
download_image	(fname, directory=".")→Path	_download(..., wantreturn=False) (L156–174)	Download asset to disk;
download_and_open_image	(fname, directory=".")→xr.Dataset	_download(fname) (L164–174)	Download + return opened dataset via open_image.
remove_path	(path)→None	_remove (L156–161)	Delete file or directory tree; no-op when absent.

generators.py

New function	Signature	Extracted from (old file)	Purpose
make_beam_fit_params	(xds)→xr.DataArray	xds_from_image_test.compare_image_block (L245–256)	Build synthetic BEAM_FIT_PARAMS DataArray from an open dataset.
create_empty_test_image	(factory, do_sky_coords=None)→xr.Dataset	make_empty_image_tests.create_image (L971–981)	Call any make_empty_* factory with canonical test coordinates.
create_bzero_bscale_fits	(outname, source_fits, bzero, bscale)→None	fits_to_xds_test._create_bzero_bscale_image (L914–925)	Write FITS file with explicit BSCALE/BZERO headers for guard tests. Also fixes original bug where _clean_data did not propagate its return value (see note below).
scale_data_for_int16	(data)→np.ndarray	fits_to_xds_test._clean_data (L927–935)	Clip + cast float array to int16 range; supports create_bzero_bscale_fits.

Bug fix in create_bzero_bscale_fits: the original _create_bzero_bscale_image called
self._clean_data(data) which internally reassigned the local variable but never returned the
result, so the float data (not int16) was passed to fits.PrimaryHDU. The extracted
create_bzero_bscale_fits uses the return value of scale_data_for_int16 so the FITS file is
correctly written with int16 pixel data.

assertions.py

New function	Signature	Extracted from (old file)	Purpose
normalize_image_coords_for_compare	(coords, factor=180*60/π)→None	casacore_to_xds_to_casacore._normalize_coords_for_compare (L409–415)	Convert direction coords from radians to arcminutes in-place so round-tripped CASA metadata can be compared with the original.
assert_image_block_equal	(xds, output_path, zarr=False)→None	xds_from_image_test.compare_image_block (L235–279)	Attach BEAM_FIT_PARAMS, write to output_path, reload a spatial block, assert equality via assert_xarray_datasets_equal.

__init__.py

Re-exports all 9 public names from the three modules. No casacore conditional needed (unlike measurement_set) because the image testing utilities only require numpy, xarray, and astropy.

2 — New test class structure for test_image.py

Classes appear in this order in the refactored file:

New class	Lines (new file)	API under test
TestLoadImage	L75–138	load_image
TestOpenImageCasa	L146–183	open_image (CASA)
TestWriteImageCasa	L191–273	write_image (CASA)
TestCasaRoundtrip	L276–538	open_image + write_image (CASA)
TestZarrRoundtrip	L540–601	open_image + write_image (zarr)
TestWriteImageZarr	L604–629	write_image (zarr, UV)
TestOpenImageFits	L632–789	open_image (FITS)
TestMakeEmptyImages	L826–857	make_empty_sky/aperture/lmuv_image

TestCasaRoundtrip and TestZarrRoundtrip test the combined open+write
behaviour; this is intentional since neither open_image nor write_image
can be validated for correctness in isolation.

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