fix(api): set plate Material from request, add null guards in LoadAsync

- NestRunner now assigns Material to plates from request.Material
- NestResponse.LoadAsync uses descriptive exceptions instead of null-forgiving operators
- Fix pre-existing FillExtents.Fill signature mismatch (add bestFits parameter)

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

OpenNest.Api/NestRunner.cs

@@ -67,7 +67,8 @@ public static class NestRunner
             var plate = new Plate(request.SheetSize)
             {
                 Thickness = request.Thickness,
-                PartSpacing = request.Spacing
+                PartSpacing = request.Spacing,
+                Material = new Material(request.Material)
             };
 
             // Build items for this pass with remaining quantities