test: add E2E gateway tests (cross-gateway messaging, interest-only)

- GatewayFixture: polls /gatewayz monitoring endpoint until both servers
  report num_gateways >= 1, replacing Task.Delay with proper synchronization
- GatewayTests: two tests covering cross-gateway delivery and interest-only
  no-delivery behaviour, using double PingAsync() instead of Task.Delay
- LeafNodeTests: replace Task.Delay(500) with sub.PingAsync()+pub.PingAsync()
  to properly fence subscription propagation without timing dependencies
- Fix GatewayManager.StartAsync to read remotes from RemoteGateways (config-
  parsed) in addition to the legacy Remotes list, enabling config-file-driven
  outbound gateway connections

src/NATS.Server/Gateways/GatewayManager.cs

@@ -210,7 +210,17 @@ public sealed class GatewayManager : IAsyncDisposable
             _options.Port = ((IPEndPoint)_listener.LocalEndPoint!).Port;
 
         _acceptLoopTask = Task.Run(() => AcceptLoopAsync(_cts.Token));
-        foreach (var remote in _options.Remotes.Distinct(StringComparer.OrdinalIgnoreCase))
+
+        // Collect outbound endpoints from both the legacy Remotes list and the
+        // config-parsed RemoteGateways list (populated by ConfigProcessor).
+        var endpoints = new HashSet<string>(_options.Remotes, StringComparer.OrdinalIgnoreCase);
+        foreach (var rgw in _options.RemoteGateways)
+        {
+            foreach (var uri in rgw.GetUrls())
+                endpoints.Add($"{uri.Host}:{uri.Port}");
+        }
+
+        foreach (var remote in endpoints)
             _ = Task.Run(() => ConnectWithRetryAsync(remote, _cts.Token));
 
         _logger.LogDebug("Gateway manager started (name={Name}, listen={Host}:{Port})",