🧪 Add unit tests for Gradient color interpolation

[johnstrand]

🎯 What: Added a unit test suite for GameUtils.Types.Gradient to address missing test coverage for color gradient interpolation and stop management.

📊 Coverage:

• Empty gradient evaluation (returns transparent black)
• Single stop gradient evaluation across all position values
• Stop sorting when adding stops out of order
• Constructor initialization using IEnumerable<(float, Color)>
• Evaluation at positions below minimum stop position and above maximum stop position
• Linear interpolation (Lerp) between stops
• Multi-stop segment evaluation

✨ Result: Ensures reliable linear interpolation, boundary handling, and stop sorting for Gradient.

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PR created automatically by Jules for task 11498178222467146015 started by @johnstrand

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