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.Server/Interfaces/ISmtpClient.cs

@@ -0,0 +1,25 @@
+using System.Threading;
+using System.Threading.Tasks;
+using MimeKit;
+
+namespace Yavsc.Interfaces
+{
+    /// <summary>
+    /// Yavsc's own SMTP client surface. Narrower than
+    /// <c>MailKit.Net.Smtp.ISmtpClient</c>: only the calls that
+    /// <c>MailSender</c> actually makes. Production wires
+    /// <c>MailKitSmtpClient</c>; tests wire a recording fake.
+    /// </summary>
+    public interface ISmtpClient : IDisposable
+    {
+        int Timeout { get; set; }
+
+        void Connect(string host, int port, MailKit.Security.SecureSocketOptions options);
+
+        void Authenticate(string userName, string password);
+
+        Task SendAsync(MimeMessage message, CancellationToken cancellationToken = default);
+
+        void Disconnect(bool quit);
+    }
+}

src/Yavsc.Server/Interfaces/ISmtpClientFactory.cs

@@ -0,0 +1,13 @@
+namespace Yavsc.Interfaces
+{
+    /// <summary>
+    /// Builds fresh <see cref="ISmtpClient"/> instances on demand.
+    /// Per-call construction is required because <c>MailSender</c>
+    /// owns its client with a <c>using</c> block: a singleton or
+    /// scoped client would race across concurrent sends.
+    /// </summary>
+    public interface ISmtpClientFactory
+    {
+        ISmtpClient CreateClient();
+    }
+}