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.Engine/FillExtents.cs

@@ -25,7 +25,8 @@ namespace OpenNest
         public List<Part> Fill(Drawing drawing, double rotationAngle = 0,
             int plateNumber = 0,
             CancellationToken token = default,
-            IProgress<NestProgress> progress = null)
+            IProgress<NestProgress> progress = null,
+            List<Engine.BestFit.BestFitResult> bestFits = null)
         {
             var pair = BuildPair(drawing, rotationAngle);
             if (pair == null)