Refactor DocSignedNotification for better encapsulation

Refactored `DocSignedNotification` to remove inheritance from
`EnvelopeReceiverDto` and introduced a required `EnvelopeReceiver`
property. Updated all usages across the codebase to align with the
new structure, including controllers, handlers, and tests.

- Improved encapsulation and reduced coupling by making
  dependencies explicit.
- Updated `AnnotationController`, `DocStatusHandler`,
  `HistoryHandler`, and `SendSignedMailHandler` to use the
  `EnvelopeReceiver` property.
- Adjusted `DocSignedNotificationTests` to reflect the new
  instantiation pattern.
- Updated XML documentation and ensured consistent access to
  `EnvelopeReceiver` properties like `EnvelopeId`, `ReceiverId`,
  and `EmailAddress`.

EnvelopeGenerator.Application/Common/Notifications/DocSigned/Handlers/DocStatusHandler.cs

@@ -31,8 +31,8 @@ public class DocStatusHandler : INotificationHandler<DocSignedNotification>
     /// <returns></returns>
     public Task Handle(DocSignedNotification notification, CancellationToken cancel) => _sender.Send(new CreateDocStatusCommand()
     {
-        EnvelopeId = notification.EnvelopeId,
-        ReceiverId = notification.ReceiverId,
+        EnvelopeId = notification.EnvelopeReceiver.EnvelopeId,
+        ReceiverId = notification.EnvelopeReceiver.ReceiverId,
         Value = notification.PsPdfKitAnnotation is PsPdfKitAnnotation annot
                     ? JsonSerializer.Serialize(annot.Instant, Format.Json.ForAnnotations)
                     : BlankAnnotationJson