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.
