Add FetchEmailUidsQuery and its handler for IMAP UIDs

Introduced `FetchEmailUidsQuery` to retrieve UIDs of matching
emails from an IMAP mailbox. Added the `FetchEmailUidsQuery`
record with properties for email account (`GetSenderQuery`)
and mail filtering (`MailSearchFilter`).

Implemented `FetchEmailUidsQueryHandler` to handle the query.
The handler validates the email account, ensures IMAP
configuration, and uses `IImapEmailService.FetchEmailUidsAsync`
to fetch UIDs. Added necessary `using` directives for
dependencies.

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

@@ -0,0 +1,42 @@
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.Common.Models.MailSearch;
+using DigitalData.MessagingService.Application.EmailAccount.Queries;
+using DigitalData.MessagingService.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.MessagingService.Application.EmailReceiving.Queries;
+
+/// <summary>
+/// Query to fetch only the UIDs of matching emails from an IMAP mailbox.
+/// </summary>
+public record FetchEmailUidsQuery : IRequest<IEnumerable<long>>
+{
+    /// <summary>
+    /// Identifies the email account to use.
+    /// </summary>
+    public required GetSenderQuery Account { get; init; }
+
+    /// <summary>
+    /// Mail query used to filter and limit the emails retrieved.
+    /// </summary>
+    public MailSearchFilter Mail { get; init; } = new();
+}
+
+public class FetchEmailUidsQueryHandler(ISender Sender, IImapEmailService ImapService) : IRequestHandler<FetchEmailUidsQuery, IEnumerable<long>>
+{
+    public async Task<IEnumerable<long>> Handle(FetchEmailUidsQuery request, CancellationToken cancellationToken)
+    {
+        var account = await Sender.Send(request.Account, cancellationToken)
+            ?? throw new NotFoundException(
+                $"No email account found for the given criteria (Id: {request.Account.Id}, Username: {request.Account.Username}).");
+
+        if (string.IsNullOrWhiteSpace(account.ImapServer))
+            throw new InvalidOperationException(
+                $"IMAP is not configured for account '{account.Username}' (Id: {account.Id}). Set ImapServer in EmailAccounts configuration.");
+
+        return await ImapService.FetchEmailUidsAsync(
+            account,
+            request.Mail,
+            cancellationToken);
+    }
+}