Refactor Attachments property and syntax

Changed the `Attachments` property type in `EnvelopeNotificationBase.cs` from `List` to `IReadOnlyList` to enhance immutability and thread safety. Updated the `FinalizeEnvelopeCommandHandler` in `FinalizeEnvelopeCommand.cs` to use a single-line syntax for `Attachments` initialization, improving code readability.

EnvelopeGenerator.Application/PdfSigning/FinalizeEnvelopeCommand.cs

@@ -98,9 +98,7 @@ public class FinalizeEnvelopeCommandHandler(IRepository<Envelope> Repo, ISender
             {
                 DocumentTitle = envelope.Title,
                 NameReceiver = er.Name,
-                Attachments = [
-                    ($"unterzeichnetes-dokument.pdf", doc)
-                    ]
+                Attachments = [($"unterzeichnetes-dokument.pdf", doc)]
             }, cancel);
         }