Add unit tests for module-level color helpers in manim/utils/color/core.py

[THE-RAF]

Closes #4740

Summary

Adds tests/module/utils/test_color_helpers.py covering the standalone helper functions exported from manim.utils.color.core (color_to_rgb, color_to_rgba, color_to_int_rgb, color_to_int_rgba, rgb_to_color, rgba_to_color, rgb_to_hex, hex_to_rgb, invert_color, color_gradient, interpolate_color, average_color, random_bright_color, random_color) and the RandomColorGenerator class.

Before this PR, only the ManimColor class itself was tested (via tests/module/utils/test_manim_color.py); the standalone helpers had zero direct test coverage.

Coverage on manim/utils/color/core.py

	Stmts	Miss	Branch	BrPart	Cover
Before	407	107	88	16	71%
After	407	68	88	15	80%

+9 pp, 39 lines newly covered. Reproducible:

pytest tests/module/utils/test_manim_color.py tests/module/utils/test_color.py \
    --cov=manim.utils.color.core --cov-branch --cov-report=term-missing \
    -p no:xdist -o addopts="" -q   # baseline

pytest tests/module/utils/test_manim_color.py tests/module/utils/test_color.py \
    tests/module/utils/test_color_helpers.py \
    --cov=manim.utils.color.core --cov-branch --cov-report=term-missing \
    -p no:xdist -o addopts="" -q   # after

Public-API coverage

Of the 18 testable symbols in core.py's __all__, 17 are now exercised by tests (94%). The only deferred symbol is get_shaded_rgb, which concerns lighting math rather than color conversion and is a better fit for a follow-up.

Test plan

• pytest tests/module/utils/test_color_helpers.py -v — 42 tests, ~0.3s, all green.
• No new runtime or dev dependencies.
• Tests follow the conventions in test_manim_color.py: numpy.testing as nt, pytest.mark.parametrize for input-variety cases, ManimColor.__eq__ for whole-color comparisons.

The file is organized by function group (parsing, hex round-trips, inversion, interpolation, aggregation, random) with one parametrized test per "linearly independent concept" (e.g., one parametrize over the 7 parsable input forms for color_to_rgb, one over the 4 corners of a hex round-trip, etc.).

[chopan050]

Thanks for these new tests! It's a very complete PR.

I just requested one change:

[chopan050]

LGTM!