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.Tests/Application/DocSignedNotificationTests.cs

@@ -53,7 +53,7 @@ public class DocSignedNotificationTests : TestBase
 
         var annots = Services.GetRequiredService<PsPdfKitAnnotation>();
 
-        var docSignedNtf = new DocSignedNotification(envRcvDto) { PsPdfKitAnnotation = annots };
+        var docSignedNtf = new DocSignedNotification { EnvelopeReceiver = envRcvDto, PsPdfKitAnnotation = annots };
 
         var sendSignedMailHandler = Host.Services.GetRequiredService<SendSignedMailHandler>();