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.

tests/DigitalData.MessagingService.Tests/Integration/EmailSenderUrlOverloadTests.cs

@@ -57,7 +57,7 @@ public sealed class EmailSenderUrlOverloadTests
     {
         var email = new Email
         {
-            Recipient = "url-overload-test@example.com",
+            Recipients = ["url-overload-test@example.com"],
             Subject = "URL Overload Integration Test",
             Body = "<p>Sent after URL-based connection.</p>",
             IsHtml = true