test: add unit tests for rgb/hex color conversion helpers

[eeshsaxena]

Description

The rgb/hex color-conversion helpers in _plotly_utils/colors were not directly covered. This adds tests/test_core/test_colors/test_color_conversions.py covering:

• hex_to_rgb for basic hex values
• label_rgb formatting a tuple and unlabel_rgb parsing it back (and their round-trip)
• find_intermediate_color at the midpoint, at both endpoints, and with colortype="rgb"

No source changes.

Verification

pytest tests/test_core/test_colors/test_color_conversions.py - 7 passed. black clean.

[emilykl]

We don't use this pattern in our Pytest tests

Suggested change

	if __name__ == "__main__":
	raise SystemExit(pytest.main([__file__, "-v"]))

[emilykl]

Suggested change

import pytest

[emilykl]

@eeshsaxena Thanks for the PR!

By convention, all tests for _plotly_utils are located in tests/test_plotly_utils/.

Can you move this test file to tests/test_plotly_utils/colors/test_color_conversions.py?

You may need to add a tests/test_plotly_utils/colors/__init__.py file as well.

Otherwise, looks good and we're happy to merge following that change.