Add methods for fetching email UIDs and emails by UID

Added `FetchEmailUidsAsync` to `IImapEmailService` for retrieving UIDs of emails matching a filter, optimizing scenarios where only identifiers are needed. Added `FetchEmailByUidAsync` to fetch a single email by UID, with optional attachment handling.

Implemented `FetchEmailUidsAsync` in `LimilabsImapEmailService` to connect to the IMAP server, construct search criteria, retrieve UIDs, and handle sorting and result limits. Added robust error handling for authentication and other failures.

Implemented `FetchEmailByUidAsync` in `LimilabsImapEmailService` to fetch email content and flags for a specific UID, map the data to `ReceivedEmailContext`, and handle errors with logging for non-critical failures.

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

@@ -19,6 +19,24 @@ public interface IImapEmailService
         MailSearchFilter filter,
         CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Fetches only the UIDs of messages matching the specified filter.
+    /// </summary>
+    Task<IEnumerable<long>> FetchEmailUidsAsync(
+        EmailAccountDto account,
+        MailSearchFilter filter,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Fetches a single email by its UID.
+    /// </summary>
+    Task<ReceivedEmailContext?> FetchEmailByUidAsync(
+        EmailAccountDto account,
+        long uid,
+        string folder = "INBOX",
+        bool withAttachments = false,
+        CancellationToken cancellationToken = default);
+
     /// <summary>
     /// Marks a message as seen (read) on the server.
     /// </summary>