Refactor IMAP sync caching and improve thread safety

Replaced `IMemoryCache` with a thread-safe `ConcurrentDictionary`
for managing IMAP sync dates in `LimilabsImapEmailService`. Added
`GetLastImapSyncDate` and `SetLastImapSyncDate` methods to handle
cache operations. Updated the `IImapEmailService` interface to
include `GetLastImapSyncDate`. Simplified the `MarkAsSeenAsync`
method signature for better readability. Introduced a private
record type `ImapCacheKey` to encapsulate cache keys.

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

@@ -29,5 +29,13 @@ public interface IImapEmailService
         long uid,
         string folder = "INBOX",
         CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets the last IMAP sync date for the specified account and folder.
+    /// </summary>
+    /// <param name="accountId"></param>
+    /// <param name="folder"></param>
+    /// <returns></returns>
+    DateTime? GetLastImapSyncDate(int accountId, string folder = "INBOX");
 }
 #endif