fix: only log session closure metric if previously established

- Add condition to ensure `_metrics.SessionClosed()` is called only when the session was in an established state before transitioning to Idle.
- Prevents misleading metrics logging for sessions that didn’t reach the established state.

Author: ruhex

BGPLite.Server/BgpSession.cs

@@ -193,8 +193,10 @@ public async Task RunAsync(CancellationToken cancellationToken = default)
         }
         finally
         {
+            var wasEstablished = _state == BgpFsmState.Established;
             TransitionTo(BgpFsmState.Idle);
-            _metrics.SessionClosed();
+            if (wasEstablished)
+                _metrics.SessionClosed();
             _metrics.PeerDisconnected();
             _logger.LogInformation("SessionClosed with {Peer}", _peerConfig.Address);
         }