feat(smtp): UpdateSmtpConfigCommand carries TlsMode + Credentials

Add two optional nullable fields (TlsMode, Credentials) to the
UpdateSmtpConfigCommand record. The handler applies preserve-if-null
semantics: an update that omits a field leaves the existing value
intact, so existing 5-arg callers remain non-breaking.

tests/ScadaLink.ManagementService.Tests/ManagementActorTests.cs

@@ -1002,6 +1002,72 @@ public class ManagementActorTests : TestKit, IDisposable
         Assert.Contains(envelope.CorrelationId, response.Error);
     }
 
+    // ========================================================================
+    // UpdateSmtpConfig — TlsMode + Credentials plumbing (preserve-if-null)
+    // ========================================================================
+
+    [Fact]
+    public void UpdateSmtpConfig_WithTlsModeAndCredentials_PersistsThem()
+    {
+        var notifRepo = Substitute.For<INotificationRepository>();
+        var existing = new Commons.Entities.Notifications.SmtpConfiguration(
+            "old.example.com", "OAuth2", "old@example.com")
+        {
+            Id = 1,
+            Port = 25,
+            TlsMode = "StartTLS",
+            Credentials = "old-secret",
+        };
+        notifRepo.GetSmtpConfigurationByIdAsync(1, Arg.Any<CancellationToken>()).Returns(existing);
+        _services.AddScoped(_ => notifRepo);
+
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new UpdateSmtpConfigCommand(1, "new.example.com", 465, "Basic", "new@example.com", "SSL", "user:pass"),
+            "Design");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.Equal(envelope.CorrelationId, response.CorrelationId);
+        Assert.Equal("SSL", existing.TlsMode);
+        Assert.Equal("user:pass", existing.Credentials);
+        Assert.Equal("new.example.com", existing.Host);
+        Assert.Equal("Basic", existing.AuthType);
+    }
+
+    [Fact]
+    public void UpdateSmtpConfig_WithNullTlsModeAndCredentials_PreservesExistingValues()
+    {
+        var notifRepo = Substitute.For<INotificationRepository>();
+        var existing = new Commons.Entities.Notifications.SmtpConfiguration(
+            "old.example.com", "OAuth2", "old@example.com")
+        {
+            Id = 1,
+            Port = 25,
+            TlsMode = "StartTLS",
+            Credentials = "old-secret",
+        };
+        notifRepo.GetSmtpConfigurationByIdAsync(1, Arg.Any<CancellationToken>()).Returns(existing);
+        _services.AddScoped(_ => notifRepo);
+
+        var actor = CreateActor();
+        var envelope = Envelope(
+            new UpdateSmtpConfigCommand(1, "new.example.com", 465, "Basic", "new@example.com"),
+            "Design");
+
+        actor.Tell(envelope);
+
+        var response = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        Assert.Equal(envelope.CorrelationId, response.CorrelationId);
+        // Omitted fields are preserved, not nulled.
+        Assert.Equal("StartTLS", existing.TlsMode);
+        Assert.Equal("old-secret", existing.Credentials);
+        // Provided fields are still updated.
+        Assert.Equal("new.example.com", existing.Host);
+        Assert.Equal("Basic", existing.AuthType);
+    }
+
     [Fact]
     public void CuratedHandlerFailure_SurfacesTheCuratedMessage()
     {