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/HistoryHandler.cs

@@ -29,13 +29,13 @@ public class HistoryHandler : INotificationHandler<DocSignedNotification>
     /// <returns></returns>
     public async Task Handle(DocSignedNotification notification, CancellationToken cancel)
     {
-        if (notification.Receiver is null)
+        if (notification.EnvelopeReceiver.Receiver is null)
             throw new InvalidOperationException($"Receiver information is missing in the notification. DocSignedNotification:\n {notification.ToJson(Format.Json.ForDiagnostics)}");
 
         await _sender.Send(new CreateHistoryCommand()
         {
-            EnvelopeId = notification.EnvelopeId,
-            UserReference = notification.Receiver.EmailAddress,
+            EnvelopeId = notification.EnvelopeReceiver.EnvelopeId,
+            UserReference = notification.EnvelopeReceiver.Receiver.EmailAddress,
             Status = EnvelopeStatus.DocumentSigned,
         }, cancel);
     }