Refactor email handling for multiple recipients

Refactored the `Email` and `OutgoingEmailEvent` classes to replace the `Recipient` property with a `Recipients` collection, enabling support for multiple recipients. Updated all related test cases, including `EmailSenderTests`, `EmailSenderUrlOverloadTests`, and `OutgoingEmailPublisherTests`, to reflect this change.

Moved the `EmailAccountDto` class and its references from the `DigitalData.MessagingService.Application.Common.Dtos` namespace to the `DigitalData.MessagingService.Publisher.Abstraction` namespace for better code organization. Updated `using` directives across affected files.

Removed unused `using` directives and updated the `Email` class's `ToEvent` method to map the new `Recipients` property. Adjusted test assertions to validate collections instead of single recipient strings.

src/presentation/DigitalData.MessagingService.API/Program.cs

@@ -1,6 +1,5 @@
 using DigitalData.MessagingService.API.Middleware;
 using DigitalData.MessagingService.Application;
-using DigitalData.MessagingService.Application.Common.Dtos;
 using DigitalData.MessagingService.Infrastructure;
 using Serilog;
 using Serilog.Ui.Core.Extensions;

src/presentation/DigitalData.MessagingService.Client/Email.cs

@@ -10,7 +10,7 @@ public record Email
     /// <summary>
     /// Recipient email address
     /// </summary>
-    public string Recipient { get; set; } = null!;
+    public IEnumerable<string> Recipients { get; set; } = null!;
 
     /// <summary>
     /// Email subject
@@ -36,7 +36,7 @@ public record Email
         return new OutgoingEmailEvent
         {
             Id = Guid.NewGuid(),
-            Recipient = Recipient,
+            Recipients = Recipients,
             Subject = Subject,
             Body = Body,
             IsHtml = IsHtml,