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

@@ -17,7 +17,7 @@ public interface IImapEmailService
     /// Set to <see langword="false"/> for a non-destructive read (uses <c>BODY.PEEK</c> internally).
     /// </param>
     /// <param name="cancellationToken">Cancellation token.</param>
-    Task<IEnumerable<ReceivedEmailContext>> FetchEmailsAsync(
+    Task<IEnumerable<ReceivedEmailDto>> FetchEmailsAsync(
         EmailAccountDto account,
         MailSearchFilter filter,
         CancellationToken cancellationToken = default);