Fix single-color palette generation

[MokiMeow]

Link to issue

Closes #5805

Description of change

Allow sample_colorscale and n_colors to return one color from the lower endpoint instead of raising ZeroDivisionError. Interpolation for larger palettes stays unchanged.

Demo

from plotly.colors import sample_colorscale, n_colors
sample_colorscale("Viridis", 1)  # ['rgb(68, 1, 84)']
n_colors((0, 0, 0), (1, 1, 1), 1)  # [(0.0, 0.0, 0.0)]

Both calls raise before the fix.

Testing strategy

Eight regression cases fail before the fix; all 14 new cases pass afterward. Colors tests: 40 passed on Python 3.8; broader Windows runs: 1898 passed on Python 3.12 and 468 on Python 3.8.

Additional information

The broader runs omit a reproduced Windows-only hierarchy test and, on 3.12, the existing Inferno_r validator failure. Changed-file lint and full formatting pass; broad Ruff retains the same 1911 baseline errors. Sdist/wheel, installed-wheel consumer and focused API documentation checks pass.

Guidelines

• I have reviewed the contributing guidelines and Code of Conduct.
• I have added a changelog entry.