fix(axis): respect offset text labelcolor (swev-id: matplotlib__matplotlib-25287)

[casey-brooks]

Summary

• ensure XAxis/YAxis offset text inherits tick.labelcolor unless it is set to inherit
• fall back to tick.color when label color is inherited
• add regression tests covering rcParam overrides, inherit fallback, and tick_params(labelcolor=...)

Issue

• closes [Bug] offsetText should use tick.labelcolor precedence over tick.color (ID: 276) #51

Reproduction Steps

1. Checkout matplotlib__matplotlib-25287
2. Set up the test environment (venv + LD_LIBRARY_PATH as described in project docs)
3. Run:

   MPLBACKEND=Agg \
   PYTHONPATH=$(pwd)/lib \
   LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib \
   .venv/bin/pytest lib/matplotlib/tests/test_axes.py -k "offset_text_uses_labelcolor_from_rc or offset_text_inherits_tick_color_when_labelcolor_inherit or offset_text_follows_tick_params_labelcolor"

Failing Test Output (before fix)

============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.2, pluggy-1.6.0
rootdir: /workspace/matplotlib
configfile: pytest.ini
collected 855 items / 852 deselected / 3 selected

lib/matplotlib/tests/test_axes.py F..                                    [100%]

=================================== FAILURES ===================================
___________________ test_offset_text_uses_labelcolor_from_rc ___________________
>               assert mcolors.to_rgba(ax.xaxis.offsetText.get_color()) == \
                    mcolors.to_rgba('red')
E                   assert (0.0, 0.0, 0.0, 1.0) == (1.0, 0.0, 0.0, 1.0)
E                     At index 0 diff: 0.0 != 1.0
E                     Use -v to get more diff

lib/matplotlib/tests/test_axes.py:6093: AssertionError
=========================== short test summary info ============================
FAILED lib/matplotlib/tests/test_axes.py::test_offset_text_uses_labelcolor_from_rc
================= 1 failed, 2 passed, 852 deselected in 1.07s ==================

Testing

• MPLBACKEND=Agg PYTHONPATH=$(pwd)/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/pytest lib/matplotlib/tests/test_axes.py -k "offset_text_uses_labelcolor_from_rc or offset_text_inherits_tick_color_when_labelcolor_inherit or offset_text_follows_tick_params_labelcolor"
• .venv/bin/flake8 lib/matplotlib/axis.py
• .venv/bin/flake8 --extend-ignore=E721 lib/matplotlib/tests/test_axes.py

[casey-brooks]

Test & Lint Summary

• ============================= test session starts ==============================
  platform linux -- Python 3.12.12, pytest-9.0.2, pluggy-1.6.0
  rootdir: /workspace/matplotlib
  configfile: pytest.ini
  collected 855 items / 852 deselected / 3 selected

lib/matplotlib/tests/test_axes.py ... [100%]

====================== 3 passed, 852 deselected in 0.34s ======================= → 3 passed, 0 failed, 0 skipped

• → no lint issues
• → no lint issues

[noa-lucent]

Looks good