Reject hex colors that are not 3 or 6 digits

[dylanpulver]

Link to issue

Closes #5728

Description of change

hex_to_rgb computed its section width as len(value) // 3 and sliced in steps of that width, so any length other than 3 or 6 produced a tuple of the wrong size instead of an error: hex_to_rgb("#12345") returned (1, 2, 3, 4, 5). An empty string raised ValueError: range() arg 3 must not be zero, which says nothing about colors. This enforces the contract the docstring already states.

Demo

Before:

>>> hex_to_rgb("#12345")
(1, 2, 3, 4, 5)
>>> hex_to_rgb("")
ValueError: range() arg 3 must not be zero

After:

>>> hex_to_rgb("#12345")
ValueError: hex color must be 3 or 6 hex digits, optionally prefixed with '#'; got '12345'

Both valid shapes are unchanged, with or without the leading #: "#ffffff", "#fff", "aabbcc" and "abc" all behave as before.

Testing strategy

Added a parametrized rejection test over "#12345", "#1", "#1234567", "" and "#", plus a case pinning that the leading # stays optional. Reverting the change fails five of them, so they are not passing vacuously.

tests/test_plotly_utils/: 1417 passed, 2 failed. Those 2 are test_fig_deepcopy pyarrow cases that fail identically on master in my environment; nothing fails only on this branch.

Not addressed here, noted in the issue: n_colors(low, high, 1) raises ZeroDivisionError because the increment is diff / (n_colors - 1). The right behaviour for n=1 is your call, so I left it alone rather than guess.

This PR was written with AI assistance (Claude Code).