fix(sessions): report a close whose kill fallback succeeded as Closed, not CloseFailed

CloseAsync rethrew the graceful-shutdown exception after a successful
Kill, so the manager faulted the session, recorded a CloseFailed fault and
surfaced CloseFailed to the caller for the ordinary hung-worker case.
docs/Sessions.md reserves CloseFailed for both shutdown and kill failing.

A successful kill now completes the close as Closed; the shutdown failure
rides back on SessionCloseResult.WorkerShutdownFailure and the manager
logs it at warning level. Both-fail still raises CloseFailed.

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManager.cs

@@ -565,6 +565,18 @@ public sealed class SessionManager : ISessionManager
         try
         {
             SessionCloseResult result = await session.CloseAsync(reason, cancellationToken).ConfigureAwait(false);
+            if (result.WorkerShutdownFailure is not null)
+            {
+                // A clean close from the session's point of view (the worker was killed and the
+                // session is Closed), but the operator should see that the worker would not stop
+                // gracefully — a run of these is the signal that workers are hanging on shutdown.
+                _logger.LogWarning(
+                    result.WorkerShutdownFailure,
+                    "Worker for session {SessionId} did not shut down gracefully and was killed; reason={Reason}.",
+                    session.SessionId,
+                    reason);
+            }
+
             if (!wasClosed && !result.AlreadyClosed && session.OpenRecorded)
             {
                 _metrics.SessionClosed();