Add tests for custom color palette feature

[pjt222]

Context

Custom color palette with color pickers was added in commits 70d8833, 11a1e70, f0efe37 but has no test coverage.

Files affected:

• R/color_utils.R - get_color_palette() with custom palette support
• R/ui_controls.R - Color picker UI components

Test Cases Needed

1. Color palette generation

   • Standard palettes (turbo, viridis, plasma, etc.)
   • Custom palette with valid hex colors
   • Inverted palettes

2. Edge cases

   • Invalid hex color format
   • Empty custom colors
   • Single color custom palette

3. UI integration

   • Color picker preview updates
   • Palette dropdown selection

Related

• Part of Issue 🧪 Expand Test Coverage #4 (Expand Test Coverage)
• Issue 🎨 Feature: Custom Color Palette Builder #8 should be closed (feature is complete)

Priority: High