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.Application/Pdf/BurnPdfCommand.cs

@@ -43,6 +43,13 @@ public class BurnPdfCommandHandler : IRequestHandler<BurnPdfCommand>
         _signRepo = signRepo;
     }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="pSourceBuffer"></param>
+    /// <param name="pInstantJSONList"></param>
+    /// <param name="envelopeId"></param>
+    /// <returns></returns>
     public byte[] BurnAnnotsToPDF(byte[] pSourceBuffer, List<string> pInstantJSONList, int envelopeId)
     {
         // read the elements of envelope with their annotations
@@ -99,10 +106,10 @@ public class BurnPdfCommandHandler : IRequestHandler<BurnPdfCommand>
 
                     var frame = element.Annotations?.FirstOrDefault(a => a.Name == "frame");
                     double frameY = element.Top - 0.5 - margin;
-                    double frameYShift = frame.Y - frameY * inchFactor;
-                    double frameXShift = frame.X - frameX * inchFactor;
+                    double frameYShift = (frame!.Y ?? default) - frameY * inchFactor;
+                    double frameXShift = (frame.X ?? default) - frameX * inchFactor;
 
-                    foreach (var annot in element.Annotations)
+                    foreach (var annot in element.Annotations!)
                     {
                         double yOffsetofFF;
                         if (!yOffsetsOfFF.TryGetValue(annot.Name, out yOffsetofFF))