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/DocSignedNotification.cs

@@ -16,25 +16,29 @@ namespace EnvelopeGenerator.Application.Common.Notifications.DocSigned;
 public record PsPdfKitAnnotation(ExpandoObject Instant, IEnumerable<AnnotationCreateDto> Structured);
 
 /// <summary>
-/// 
+/// Notification raised when a document is signed by a receiver.
 /// </summary>
-/// <param name="Original"></param>
-public record DocSignedNotification(EnvelopeReceiverDto Original) : EnvelopeReceiverDto(Original), INotification, ISendMailNotification
+public record DocSignedNotification : INotification, ISendMailNotification
 {
     /// <summary>
-    /// 
+    /// The envelope receiver information.
+    /// </summary>
+    public required EnvelopeReceiverDto EnvelopeReceiver { get; init; }
+
+    /// <summary>
+    /// The PSPDFKit annotation data.
     /// </summary>
     public PsPdfKitAnnotation? PsPdfKitAnnotation { get; init; }
 
     /// <summary>
-    /// 
+    /// Gets the email template type.
     /// </summary>
     public EmailTemplateType TemplateType => EmailTemplateType.DocumentSigned;
 
     /// <summary>
-    /// 
+    /// Gets the email address of the receiver.
     /// </summary>
-    public string EmailAddress => Receiver?.EmailAddress
+    public string EmailAddress => EnvelopeReceiver.Receiver?.EmailAddress
         ?? throw new InvalidOperationException($"Receiver is null." +
             $"DocSignedNotification:\n{this.ToJson(Format.Json.ForDiagnostics)}");
 }