Fix api test issues

[mwcraig]

It turns out that there were a number of issues in the file that downstream packages would use to test API compatibility. This PR fixes them and marks one as xfail that tests colormaps.

[Copilot]

Pull Request Overview

This PR addresses issues in the API compatibility tests by fixing marker table return properties and error conditions while also marking the colormap test as xfail. Key changes include:

• Renaming and updating marker table check functions and replacing direct attribute accesses with helper methods.
• Modifying the FITS loading test to write and read from a temporary file.
• Enhancing error matching in tests for offset operations, stretch, and cuts.

Comments suppressed due to low confidence (1)

src/astro_image_display_api/widget_api_test.py:52

• [nitpick] Consider renaming '_check_empty_marker_table_return_properties' to a more concise name like '_assert_empty_marker_table' for clarity and brevity.

def _check_empty_marker_table_return_properties(self, table):

[Copilot]

Pull Request Overview

This PR addresses API test issues by refactoring marker-related testing functions, refining exception match patterns, and marking the colormap test as xfail for downstream compatibility.

• Renamed the marker table check method for clarity
• Updated exception match patterns in several tests
• Reworked file I/O in the FITS loading test and marked colormap tests as xfail

Comments suppressed due to low confidence (1)

src/astro_image_display_api/widget_api_test.py:110

• Verify that the error message pattern 'are not convertible' matches the actual message from the underlying unit conversion logic, considering possible library updates.

with pytest.raises(u.UnitConversionError, match='are not convertible'):