Refactor: Replace EmailAttachmentContext with DTO

Replaced `EmailAttachmentContext` with `EmailAttachmentDto` across the codebase to align with the updated DTO naming convention.

- Renamed `EmailAttachmentContext` to `EmailAttachmentDto`.
- Updated property types, method signatures, and return types to use `EmailAttachmentDto`.
- Modified XML documentation references to reflect the new class name.
- Updated `ReceivedEmailContext` to `ReceivedEmailDto` and adjusted related methods and properties.
- Refactored `FetchEmailsAsync` methods and handlers to use `ReceivedEmailDto`.
- Adjusted `BuildAttachmentsAsync` and attachment handling logic in `EmailController` and `LimilabsImapEmailService`.

This refactor ensures consistency and improves code clarity while maintaining functionality.

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsEmailService.cs

@@ -111,7 +111,7 @@ public class LimilabsEmailService(
         return message.ToString();
     }
 
-    private static void AddAttachments(MailBuilder builder, IEnumerable<EmailAttachmentContext> attachments)
+    private static void AddAttachments(MailBuilder builder, IEnumerable<EmailAttachmentDto> attachments)
     {
         foreach (var attachment in attachments)
         {