fix(BurnPdfCommand): improve JSON serialization and null safety in Signature entity and BurnPdfCommandHandler

- Added System.Text.Json.Serialization namespace and [JsonIgnore] attributes to Top and Left properties in Signature entity for .NET builds
- Applied null-forgiving operator (!) and null-coalescing defaults for frame.X and frame.Y in BurnPdfCommandHandler
- Improved runtime safety and prevented unintended JSON serialization of computed properties

EnvelopeGenerator.Domain/Entities/Signature.cs

@@ -2,6 +2,7 @@
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+using System.Text.Json.Serialization;
 #if NETFRAMEWORK
 using System;
 using System.Collections.Generic;
@@ -120,13 +121,17 @@ public class Signature : ISignature, IHasReceiver
 #endif
         Annotations { get; set; }
 
-#if NETFRAMEWORK
+#if NET
+    [JsonIgnore]
+#endif
     [NotMapped]
     public double Top => Math.Round(Y, 5);
 
+#if NET
+    [JsonIgnore]
+#endif
     [NotMapped]
     public double Left => Math.Round(X, 5);
-#endif
 }
 
 #if NETFRAMEWORK