fix: ensure pyplot set_cmap handles registered keys (swev-id: matplotlib__matplotlib-25479)

[casey-brooks]

Summary

• ensure pyplot.set_cmap stores the passed registry key when provided a string
• keep colormap-object inputs writing the object's intrinsic name
• add regression coverage for rcParams behavior with both string and object inputs

Fixes #58

Testing

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/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 .venv/bin/pytest lib/matplotlib/tests/test_pyplot.py -k 'set_cmap_'
• .venv/bin/flake8 lib/matplotlib/pyplot.py lib/matplotlib/tests/test_pyplot.py

Reproduction

Pre-fix script:

import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
from matplotlib.colors import LinearSegmentedColormap

lsc = LinearSegmentedColormap.from_list('some_cmap_name', ['black', 'white'])
mpl.colormaps.register(lsc, name='my_cmap_name', force=True)
plt.set_cmap('my_cmap_name')
plt.imshow(np.arange(4).reshape(2, 2))

Observed failure (excerpt):

ValueError: 'some_cmap_name' is not a valid value for cmap; supported values are ... 'my_cmap_name', ...
  File ".../matplotlib/cm.py", line 722, in _ensure_cmap
    _api.check_in_list(sorted(_colormaps), cmap=cmap_name)

Post-fix confirmation:

imshow succeeded; rc image.cmap = my_cmap_name

[casey-brooks]

Test & Lint Summary

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/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 .venv/bin/pytest lib/matplotlib/tests/test_pyplot.py -k 'set_cmap_' → 2 passed, 26 deselected
• .venv/bin/flake8 lib/matplotlib/pyplot.py lib/matplotlib/tests/test_pyplot.py → no issues

[noa-lucent]

Looks good. The change ensures string inputs write the registered key into rcParams while colormap objects continue to use their intrinsic names, and the new regression tests cover both code paths.