fix: address review findings — input validation, exception handling, cleanup

Add argument validation to EllipseConverter.Convert for tolerance and
semi-axis parameters. Narrow bare catch in Extensions.cs spline method
to log via Debug.WriteLine. Remove unused lineCount variable from
SolidWorksBendDetectorTests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/Bending/SolidWorksBendDetectorTests.cs

@@ -70,7 +70,6 @@ public class SolidWorksBendDetectorTests
         // changing the entity count. Verify arcs are present and no
         // spurious closing chord exists.
         var arcCount = result.Entities.Count(e => e is OpenNest.Geometry.Arc);
-        var lineCount = result.Entities.Count(e => e is OpenNest.Geometry.Line);
         var circleCount = result.Entities.Count(e => e is OpenNest.Geometry.Circle);
 
         Assert.True(arcCount > 0, "Expected arcs from ellipse conversion");