Announce a session's end when it is actually over, and for closes too

The phone's notification kept saying '1 shell connected' after the
shell was gone, and both close routes were at fault. A shell exiting on
its own raised SessionEnded from inside its run's finally block — where
the run task is by definition not yet complete, so the LiveSessionCount
the keep-alive reads still counted the dead shell, and nothing fired
later to correct it. A tab closed by hand announced nothing at all, by
a recorded decision that assumed every subscriber was the closer; the
keep-alive is not, and a close it never heard about left the
notification claiming a shell over nothing.

The end is now announced from a continuation after the run completes,
and CloseSessionAsync announces after its own drain — every subscriber
was already a reconcile-to-reality handler, so the echo the old remark
feared costs nothing. Shutdown stays silent: it is dismantling the
subscribers along with the sessions.

src/DodoSSH.Client.Shell/ViewModels/MainWindowViewModel.cs

@@ -1936,9 +1936,9 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
             return;
         }
 
-        // Removed first, so the workspace's SessionEnded — which fires as the pump unwinds — finds no tab to
-        // mark dead and does nothing. The alternative ordering leaves a window in which a tab that is on its
-        // way out is repainted as disconnected.
+        // Removed first, so the workspace's SessionEnded — announced once the close below has fully drained
+        // — finds no tab to mark dead and does nothing here. The alternative ordering leaves a window in
+        // which a tab that is on its way out is repainted as disconnected.
         var index = Tabs.IndexOf(tab);
         Tabs.Remove(tab);