Description

Rejects negative inputs in SumOfSquares instead of returning the mathematically incorrect result 3.

The public method now throws IllegalArgumentException with a clear message, and its Javadoc documents the behavior.

Testing

• Added a regression test for negative input
• Compiled the implementation directly with javac
• Verified the existing one-, two-, three-, and four-square cases
• Verified that negative input throws IllegalArgumentException

Closes #7538

Checklist

• Filenames use PascalCase
• Java naming conventions are followed
• The existing algorithm documentation link is preserved
• The behavior is covered by a corresponding test
• The change follows the existing formatting style

AI assistance was used to prepare this focused fix. I reviewed and validated the implementation.