Pass cancellationToken to next() in pipeline behaviors

Updated the `Handle` method in multiple classes implementing
`IPipelineBehavior<SignCommand, Unit>` to pass the `cancellationToken`
parameter to the `next()` method. This change ensures consistent
propagation of the `cancellationToken` through the pipeline, enabling
proper handling of cancellation requests during asynchronous operations.

Modified files:
- AnnotationBehavior.cs
- DocStatusBehavior.cs
- HistoryBehavior.cs
- SendSignedMailBehavior.cs

EnvelopeGenerator.Application/Signatures/Behaviors/AnnotationBehavior.cs

@@ -36,6 +36,6 @@ public class AnnotationBehavior : IPipelineBehavior<SignCommand, Unit>
         if (request.PsPdfKitAnnotation is PsPdfKitAnnotation annot)
             await _repo.CreateAsync(annot.Structured, cancellationToken);
 
-        return await next();
+        return await next(cancellationToken);
     }
 }

EnvelopeGenerator.Application/Signatures/Behaviors/DocStatusBehavior.cs

@@ -45,6 +45,6 @@ public class DocStatusBehavior : IPipelineBehavior<SignCommand, Unit>
                         : BlankAnnotationJson
         }, cancellationToken);
 
-        return await next();
+        return await next(cancellationToken);
     }
 }

EnvelopeGenerator.Application/Signatures/Behaviors/HistoryBehavior.cs

@@ -42,6 +42,6 @@ public class HistoryBehavior : IPipelineBehavior<SignCommand, Unit>
             Status = EnvelopeStatus.DocumentSigned,
         }, cancellationToken);
 
-        return await next();
+        return await next(cancellationToken);
     }
 }

EnvelopeGenerator.Application/Signatures/Behaviors/SendSignedMailBehavior.cs

@@ -79,7 +79,7 @@ public class SendSignedMailBehavior : IPipelineBehavior<SignCommand, Unit>
 
         await _emailOutRepo.CreateAsync(emailOut, cancellationToken);
 
-        return await next();
+        return await next(cancellationToken);
     }
 
     private Dictionary<string, string> CreatePlaceHolders(SignCommand request)