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/Common/Interfaces/Services/Notification/EnvelopeNotificationBase.cs

@@ -15,5 +15,5 @@ public abstract record EnvelopeNotificationBase
     /// <summary>
     /// 
     /// </summary>
-    public List<(string FileName, byte[] Content)>? Attachments { get; init; }
+    public IReadOnlyList<(string FileName, byte[] Content)>? Attachments { get; init; }
 }