refactor: use 'length' instead of 'height' in nest JSON format

Rename SizeDto.Height to SizeDto.Length so the serialized JSON
uses "width"/"length" which is more natural for plate materials.
The core Size struct still uses Height internally.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.IO/NestFormat.cs

@@ -72,7 +72,7 @@ namespace OpenNest.IO
         public record SizeDto
         {
             public double Width { get; init; }
-            public double Height { get; init; }
+            public double Length { get; init; }
         }
 
         public record MaterialDto