fix(sessions): account SessionClosed for sweep-driven closes

TryBeginCloseIfExpired set _closeStarted when it reserved a session, so the
sweep's own CloseAsync reported AlreadyClosed: true and CloseSessionCoreAsync
skipped SessionClosed: lease-expired, faulted-reaped and detach-grace-expired
closes never left the sessions.open gauge and under-counted sessions.closed.
The reservation now writes Closing only; the first CloseAsync to take the
close lock does the accounting exactly once.

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

@@ -1822,6 +1822,15 @@ public sealed class GatewaySession
     ///         committing to <c>Closing</c>, so a reattach that wins the race leaves the session
     ///         in <c>Ready</c> and usable.
     ///     </para>
+    ///     <para>
+    ///         This is a <em>reservation</em>, not the close itself: it flips the state so no
+    ///         reattach can revive the session and no other sweep pass re-selects it, but it
+    ///         deliberately leaves <c>_closeStarted</c> untouched. The sweep's own
+    ///         <see cref="CloseAsync"/> that follows is the first close to run, and must report
+    ///         <c>AlreadyClosed: false</c> so the manager accounts <c>SessionClosed</c> for it;
+    ///         setting <c>_closeStarted</c> here made that close look like a second, concurrent one
+    ///         and every sweep-driven close went unaccounted.
+    ///     </para>
     /// </remarks>
     internal bool TryBeginCloseIfExpired(DateTimeOffset now, out bool alreadyClosing)
     {
@@ -1843,7 +1852,6 @@ public sealed class GatewaySession
             }
 
             alreadyClosing = _closeStarted;
-            _closeStarted = true;
             _state = SessionState.Closing;
             return true;
         }