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/SendSignedMailHandler.cs

@@ -31,7 +31,7 @@ public class SendSignedMailHandler : SendMailHandler<DocSignedNotification>
     protected override void ConfigureEmailOut(DocSignedNotification notification, EmailOut emailOut)
     {
         emailOut.ReferenceString = notification.EmailAddress;
-        emailOut.ReferenceId = notification.ReceiverId;
+        emailOut.ReferenceId = notification.EnvelopeReceiver.ReceiverId;
     }
 
     /// <summary>
@@ -42,11 +42,11 @@ public class SendSignedMailHandler : SendMailHandler<DocSignedNotification>
     {
         var placeHolders = new Dictionary<string, string>()
         {
-            { "[NAME_RECEIVER]", notification.Name ?? string.Empty },
-            { "[DOCUMENT_TITLE]", notification.Envelope?.Title ?? string.Empty },
+            { "[NAME_RECEIVER]", notification.EnvelopeReceiver.Name ?? string.Empty },
+            { "[DOCUMENT_TITLE]", notification.EnvelopeReceiver.Envelope?.Title ?? string.Empty },
         };
 
-        if (notification.Envelope.IsReadAndConfirm())
+        if (notification.EnvelopeReceiver.Envelope.IsReadAndConfirm())
         {
             placeHolders["[SIGNATURE_TYPE]"] = "Lesen und bestätigen";
             placeHolders["[DOCUMENT_PROCESS]"] = string.Empty;