Refactor: Replace EmailAccountDto with EmailAccount

Replaced the `EmailAccountDto` class with the `EmailAccount` class across the codebase to consolidate the `EmailAccount` entity into the domain layer. Updated namespaces, method signatures, property types, and test cases to reflect this change.

Moved `EmailAccount` from `DigitalData.MessagingService.Application.Common.Dto` to `DigitalData.MessagingService.Domain.Entities`. Updated XML documentation and removed redundant project file entries. Adjusted namespaces for related queries, validators, and commands to align with the new structure.

These changes improve separation of concerns and align with domain-driven design principles.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IImapEmailService.cs

@@ -1,6 +1,7 @@
 #if NET
 using DigitalData.MessagingService.Application.Common.Dto;
 using DigitalData.MessagingService.Application.Common.Dto.MailSearch;
+using DigitalData.MessagingService.Domain.Entities;
 
 namespace DigitalData.MessagingService.Application.Common.Interfaces;
 
@@ -19,7 +20,7 @@ public interface IImapEmailService
     /// </param>
     /// <param name="cancellationToken">Cancellation token.</param>
     Task<IEnumerable<ReceivedEmailDto>> FetchEmailsAsync(
-        EmailAccountDto account,
+        EmailAccount account,
         MailSearchFilter filter,
         CancellationToken cancellationToken = default);
 
@@ -27,7 +28,7 @@ public interface IImapEmailService
     /// Marks a message as seen (read) on the server.
     /// </summary>
     Task MarkAsSeenAsync(
-        EmailAccountDto account,
+        EmailAccount account,
         long uid,
         string folder = "INBOX",
         CancellationToken cancellationToken = default);