🧪 test: improve test coverage for utility functions

[BTawaifi]

🎯 What: The testing gap addressed: Missing proper test script and lack of complete branch coverage for src/utils.mjs. Added a node:test execution script with coverage, and added missing tests for error-handling and fallback paths in both rgbaToHexAndAlpha and hexToRgba functions.

📊 Coverage: Now tests what happens when hexToRgba receives missing or non-string inputs (null, '', 123456, 'rgb...'), and similarly for rgbaToHexAndAlpha receiving invalid inner structures ('rgba(,,)', 'rgba(abc)').

✨ Result: Test script is functional. Coverage for src/utils.mjs has increased from ~94% (lines) and 75% (branches) to 100% across the board (lines, branches, functions).

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PR created automatically by Jules for task 15795159313837192962 started by @BTawaifi

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