Introduce Yavsc.Interfaces.ISmtpClient and a recording test fake

Narrow ISmtpClient to the four operations MailSender actually uses,
behind a Yavsc.Interfaces.ISmtpClientFactory. Production wires
MailKitSmtpClient (SmtpClientFactory); tests wire a recording fake
(RecordingSmtpClientFactory). The fake is pre-registered in
WebServerFixture so SMTP calls are short-circuited; the EMailling
test now asserts the Connect -> Authenticate -> Send -> Disconnect
sequence. Yavsc.Abstract stays free of MailKit/MimeKit.

src/Yavsc.Org.Tests/Fakes/RecordingSmtpClient.cs

@@ -0,0 +1,71 @@
+using MailKit.Security;
+using MimeKit;
+using Yavsc.Interfaces;
+
+namespace Yavsc.Org.Tests.Fakes
+{
+    /// <summary>
+    /// In-memory test double for <see cref="ISmtpClient"/>. Records
+    /// every call for assertion and short-circuits the SMTP
+    /// roundtrip.
+    /// </summary>
+    public sealed class RecordingSmtpClient : ISmtpClient
+    {
+        public List<RecordingSmtpCall> Calls { get; } = new();
+
+        public int Timeout { get; set; }
+        public string? LastConnectedHost { get; private set; }
+        public int LastConnectedPort { get; private set; }
+        public SecureSocketOptions LastConnectedOptions { get; private set; }
+        public string? LastAuthenticatedUser { get; private set; }
+        public MimeMessage? LastSentMessage { get; private set; }
+        public bool WasDisconnected { get; private set; }
+        public bool Disposed { get; private set; }
+
+        public void Connect(string host, int port, SecureSocketOptions options)
+        {
+            LastConnectedHost = host;
+            LastConnectedPort = port;
+            LastConnectedOptions = options;
+            Calls.Add(new RecordingSmtpCall(RecordingSmtpCallKind.Connect, host, port, options));
+        }
+
+        public void Authenticate(string userName, string password)
+        {
+            LastAuthenticatedUser = userName;
+            Calls.Add(new RecordingSmtpCall(RecordingSmtpCallKind.Authenticate, userName, null, null));
+        }
+
+        public Task SendAsync(MimeMessage message, CancellationToken cancellationToken = default)
+        {
+            LastSentMessage = message;
+            Calls.Add(new RecordingSmtpCall(RecordingSmtpCallKind.Send, message.Subject, null, null));
+            return Task.CompletedTask;
+        }
+
+        public void Disconnect(bool quit)
+        {
+            WasDisconnected = quit;
+            Calls.Add(new RecordingSmtpCall(RecordingSmtpCallKind.Disconnect, null, null, null));
+        }
+
+        public void Dispose()
+        {
+            Disposed = true;
+        }
+    }
+
+    public enum RecordingSmtpCallKind
+    {
+        Connect,
+        Authenticate,
+        Send,
+        Disconnect,
+    }
+
+    public readonly record struct RecordingSmtpCall(
+        RecordingSmtpCallKind Kind,
+        string? HostOrSubjectOrUser,
+        int? Port,
+        SecureSocketOptions? Options);
+}