Fix reliability findings

src/MxGateway.Server/Grpc/EventStreamService.cs

@@ -66,6 +66,8 @@ public sealed class EventStreamService(
         {
             await streamCts.CancelAsync().ConfigureAwait(false);
             subscriber.Dispose();
+            Interlocked.Exchange(ref streamQueueDepth, 0);
+            metrics.SetGrpcEventStreamQueueDepth(0);
             metrics.StreamDisconnected("Detached");
 
             try

src/MxGateway.Server/Metrics/GatewayMetrics.cs

@@ -101,6 +101,17 @@ public sealed class GatewayMetrics : IDisposable
         _sessionsClosedCounter.Add(1);
     }
 
+    public void SessionRemoved()
+    {
+        lock (_syncRoot)
+        {
+            if (_openSessions > 0)
+            {
+                _openSessions--;
+            }
+        }
+    }
+
     public void WorkerStarted(TimeSpan startupDuration)
     {
         lock (_syncRoot)

src/MxGateway.Server/Sessions/SessionManager.cs

@@ -184,8 +184,11 @@ public sealed class SessionManager : ISessionManager
                     exception,
                     "Graceful shutdown failed for session {SessionId}; killing worker.",
                     session.SessionId);
-                session.KillWorker(GatewayShutdownReason);
-                await RemoveSessionAsync(session).ConfigureAwait(false);
+                if (_registry.TryGet(session.SessionId, out _))
+                {
+                    session.KillWorker(GatewayShutdownReason);
+                    await RemoveSessionAsync(session).ConfigureAwait(false);
+                }
             }
         }
     }
@@ -210,7 +213,13 @@ public sealed class SessionManager : ISessionManager
         catch (Exception exception)
         {
             session.MarkFaulted(exception.Message);
+            if (!wasClosed)
+            {
+                _metrics.SessionRemoved();
+            }
+
             _metrics.Fault(SessionManagerErrorCode.CloseFailed.ToString());
+            await RemoveSessionAsync(session).ConfigureAwait(false);
             throw new SessionManagerException(
                 SessionManagerErrorCode.CloseFailed,
                 $"Failed to close session {session.SessionId}.",