🧪 Add unit tests for Gradient class

[johnstrand]

🎯 What: Adds unit test coverage for Gradient class and its Evaluate, AddStop, and constructor methods.

📊 Coverage: Empty gradient evaluation, single stop, multi-stop linear interpolation, out-of-bounds position clamping, unsorted stop auto-sorting, constructor initialization, and method chaining.

✨ Result: Ensures reliable color gradient evaluation across edge cases and standard usage.

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PR created automatically by Jules for task 6341413901940563844 started by @johnstrand

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