Add support for optional email attachments fetching

Introduced a `withAttachments` parameter to the `FetchEmailsAsync` method in `IImapEmailService` and related layers, allowing callers to include or exclude attachment data when fetching emails. Updated `FetchEmailsQuery` and `FetchEmailsQueryHandler` to propagate this parameter.

Refactored `LimilabsImapEmailService` to conditionally process attachments and inline visuals based on the `withAttachments` flag, improving performance when attachments are not required. Replaced `Flag.Unseen` with `Expression.HasFlag(Flag.Unseen)` for better criteria handling. Cleaned up attachment-processing logic for improved readability and maintainability.

src/core/DigitalData.MessagingService.Application/EmailReceiving/Queries/FetchEmailsQuery.cs

@@ -21,6 +21,12 @@ public record FetchEmailsQuery : IRequest<IEnumerable<ReceivedEmailContext>>
     /// Mail query used to filter and limit the emails retrieved.
     /// </summary>
     public MailSearchFilter Mail { get; init; } = new();
+
+    /// <summary>
+    /// When <see langword="true"/>, attachment data is included in the results; otherwise attachments are omitted.
+    /// Defaults to <see langword="false"/>.
+    /// </summary>
+    public bool WithAttachments { get; init; } = false;
 }
 
 public class FetchEmailsQueryHandler(
@@ -40,6 +46,7 @@ public class FetchEmailsQueryHandler(
         return await ImapService.FetchEmailsAsync(
             account,
             request.Mail,
+            request.WithAttachments,
             cancellationToken);
     }
 }