fix race

Author: KirillKurdyukov

src/Ydb.Sdk/src/Ado/Session/PoolingSessionSource.cs

@@ -157,6 +157,8 @@ private async ValueTask<ISession> RentAsync(CancellationToken cancellationToken)
 
             if (IsDisposed)
             {
+                Interlocked.Decrement(ref _numSessions);
+
                 break;
             }