Add FetchEmailByUidQuery with handler and validation

Introduced `FetchEmailByUidQuery` to fetch a single email by its
UID from an IMAP mailbox. The query includes properties for
email account, UID, folder, and attachment inclusion, with a
fluent method `WithUid(long uid)` to set the UID.

Added `FetchEmailByUidQueryHandler` to process the query,
leveraging `ISender` for account retrieval and `IImapEmailService`
for email fetching. Included error handling for missing accounts
and misconfigured IMAP settings.

Implemented `FetchEmailByUidQueryValidator` to validate the
query, ensuring the `Account` is not null, `Uid` is set and
greater than 0, and `Folder` is not empty.

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

@@ -0,0 +1,62 @@
+using DigitalData.MessagingService.Abstraction;
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.EmailAccount.Queries;
+using DigitalData.MessagingService.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.MessagingService.Application.EmailReceiving.Queries;
+
+/// <summary>
+/// Query to fetch a single email by its UID from an IMAP mailbox.
+/// </summary>
+public record FetchEmailByUidQuery : IRequest<ReceivedEmailContext?>
+{
+    /// <summary>
+    /// Identifies the email account to use.
+    /// </summary>
+    public required GetSenderQuery Account { get; init; }
+
+    /// <summary>
+    /// UID of the message to fetch.
+    /// </summary>
+    internal long? Uid { get; private set; } = null;
+
+    public FetchEmailByUidQuery WithUid(long uid)
+    {
+        Uid = uid;
+        return this;
+    }
+
+    /// <summary>
+    /// Mailbox folder the message resides in (default: "INBOX").
+    /// </summary>
+    public string Folder { get; init; } = "INBOX";
+
+    /// <summary>
+    /// When <see langword="true"/>, attachment data is included in the result.
+    /// </summary>
+    public bool WithAttachments { get; init; } = false;
+}
+
+public class FetchEmailByUidQueryHandler(
+    ISender Sender,
+    IImapEmailService ImapService) : IRequestHandler<FetchEmailByUidQuery, ReceivedEmailContext?>
+{
+    public async Task<ReceivedEmailContext?> Handle(FetchEmailByUidQuery 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.FetchEmailByUidAsync(
+            account,
+            (long)request.Uid!,
+            request.Folder,
+            request.WithAttachments,
+            cancellationToken);
+    }
+}

src/core/DigitalData.MessagingService.Application/EmailReceiving/Validators/FetchEmailByUidQueryValidator.cs

@@ -0,0 +1,27 @@
+using DigitalData.MessagingService.Application.EmailReceiving.Queries;
+using FluentValidation;
+
+namespace DigitalData.MessagingService.Application.EmailReceiving.Validators;
+
+/// <summary>
+/// Validates a <see cref="FetchEmailByUidQuery"/> before it is handled by <see cref="FetchEmailByUidQueryHandler"/>.
+/// </summary>
+public class FetchEmailByUidQueryValidator : AbstractValidator<FetchEmailByUidQuery>
+{
+    public FetchEmailByUidQueryValidator()
+    {
+        RuleFor(x => x.Account)
+            .NotNull()
+            .WithMessage("Account query must not be null.");
+
+        RuleFor(x => x.Uid)
+            .NotNull()
+            .WithMessage("UID must be provided. Use WithUid() to set the UID before dispatching the query.")
+            .GreaterThan(0)
+            .WithMessage("UID must be greater than 0.");
+
+        RuleFor(x => x.Folder)
+            .NotEmpty()
+            .WithMessage("Folder must not be empty.");
+    }
+}