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/Services/SmtpClientFactory.cs

@@ -0,0 +1,14 @@
+using Yavsc.Interfaces;
+
+namespace Yavsc.Services
+{
+    /// <summary>
+    /// Production <see cref="ISmtpClientFactory"/>: hands out a
+    /// fresh <see cref="MailKitSmtpClient"/> per call. Stateless;
+    /// safe to register as a singleton.
+    /// </summary>
+    public sealed class SmtpClientFactory : ISmtpClientFactory
+    {
+        public ISmtpClient CreateClient() => new MailKitSmtpClient();
+    }
+}