Preserve hinted color spaces when deriving XObject image modes

[eerose2603]

Summary

• preserve the preferred mode hinted by /ColorSpace when resolving nested image color spaces in _get_mode_and_invert_color, keeping CMYK hints inverted when used as a fallback: git_url="https://github.com/eerose2603/pypdf/blob/main/pypdf/_xobj_image_helpers.py#L367-L400"}​
• extend regression coverage to ensure DeviceRGB, DeviceGray, and DeviceCMYK hints remain authoritative even when the color space object collapses to Null​:codex-file-citation :git_url="https://github.com/eerose2603/pypdf/blob/main/tests/test_xobject_image_helpers.py#L170-L197"}​

Testing

• python -m pytest tests/test_xobject_image_helpers.py -k "prefers"

[stefan6419846]

Thanks for the PR. Could you please elaborate on the reasons for this change? Additionally, please have a look at the CI failures for us to continue.