Fix unlabel_rgb raising on a list of colors

[vineoy]

Link to issue

Fixes #3054.

Description of change

The \unlabel_rgb\ docstring states it takes either a single 'rgb(a, b, c)'\ color or a list of such colors, but passing a list crashed with \ValueError: could not convert string to float: ''\ because the character-by-character parser ran over the list itself.

The function now maps over lists/tuples elementwise, so \unlabel_rgb(sequential.YlOrRd)\ returns a list of (r, g, b)\ tuples as documented. Single-string behavior is unchanged (all existing callers pass single strings).

Testing strategy

• New regression test \ est_unlabel_rgb_parses_list\ (including the empty-list edge) alongside the existing test: add unit tests for rgb/hex color conversion helpers #5659 color-helper tests.
• \CHANGELOG.md\ entry added under Unreleased/Fixed.
• Local runs: all 19 tests in \ ests/test_plotly_utils/colors/\ pass; pinned
  uff==0.11.12 check\ clean; verified the exact repro from the issue.

[camdecoster]

Thanks for the PR! We'll review this and follow up.