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();
+    }
+}

src/Yavsc.Server/Services/MailKitSmtpClient.cs

@@ -0,0 +1,53 @@
+using MailKit.Security;
+using MimeKit;
+using Yavsc.Interfaces;
+
+namespace Yavsc.Services
+{
+    /// <summary>
+    /// Production adapter from <see cref="Yavsc.Interfaces.ISmtpClient"/>
+    /// to MailKit's <c>SmtpClient</c>. Stateless; the underlying
+    /// MailKit client is created per <c>Connect</c> cycle by the
+    /// factory and disposed by <c>MailSender</c>'s <c>using</c>.
+    /// </summary>
+    public sealed class MailKitSmtpClient : ISmtpClient
+    {
+        private readonly MailKit.Net.Smtp.SmtpClient _client;
+
+        public MailKitSmtpClient()
+        {
+            _client = new MailKit.Net.Smtp.SmtpClient();
+        }
+
+        public int Timeout
+        {
+            get => _client.Timeout;
+            set => _client.Timeout = value;
+        }
+
+        public void Connect(string host, int port, SecureSocketOptions options)
+        {
+            _client.Connect(host, port, options);
+        }
+
+        public void Authenticate(string userName, string password)
+        {
+            _client.Authenticate(userName, password);
+        }
+
+        public Task SendAsync(MimeMessage message, CancellationToken cancellationToken = default)
+        {
+            return _client.SendAsync(message, cancellationToken);
+        }
+
+        public void Disconnect(bool quit)
+        {
+            _client.Disconnect(quit);
+        }
+
+        public void Dispose()
+        {
+            _client.Dispose();
+        }
+    }
+}

src/Yavsc.Server/Services/MailSender.cs

@@ -1,11 +1,11 @@
 using System.Net;
-using MailKit.Net.Smtp;
 using MailKit.Security;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using MimeKit;
 using Microsoft.AspNetCore.Identity;
 using Yavsc.Interface;
+using Yavsc.Interfaces;
 using Yavsc.Settings;
 using Yavsc.Models;
 using Microsoft.AspNetCore.Identity.UI.Services;
@@ -16,21 +16,26 @@ namespace Yavsc.Services
 {
     public class MailSender : IEmailSender<ApplicationUser>, IEmailSender, ITrueEmailSender
     {
+
         private readonly IStringLocalizer<MailSender> localizer;
         readonly SiteSettings siteSettings;
         readonly SmtpSettings smtpSettings;
         private readonly ILogger logger;
+        private readonly ISmtpClientFactory _smtpClientFactory;
+
         public MailSender(
             IOptions<SiteSettings> sitesOptions,
             IOptions<SmtpSettings> smtpOptions,
             ILoggerFactory loggerFactory,
-            IStringLocalizer<MailSender> localizer
+            IStringLocalizer<MailSender> localizer,
+            ISmtpClientFactory smtpClientFactory
             )
         {
             this.localizer = localizer;
             siteSettings = sitesOptions.Value;
             smtpSettings = smtpOptions.Value;
             logger = loggerFactory.CreateLogger<MailSender>();
+            _smtpClientFactory = smtpClientFactory;
         }
 
         public Task SendConfirmationLinkAsync(ApplicationUser user, string email, string confirmationLink)
@@ -69,7 +74,7 @@ namespace Yavsc.Services
             msg.MessageId = MimeKit.Utils.MimeUtils.GenerateMessageId(
                 siteSettings.Authority
             );
-            using (SmtpClient sc = new())
+            using ISmtpClient sc = _smtpClientFactory.CreateClient();
             {
                 sc.Timeout = 30000;
                 sc.Connect(

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();
+    }
+}