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

tests/NATS.E2E.Tests/Infrastructure/GatewayFixture.cs

@@ -0,0 +1,111 @@
+using System.Net.Http.Json;
+using NATS.Client.Core;
+
+namespace NATS.E2E.Tests.Infrastructure;
+
+public sealed class GatewayFixture : IAsyncLifetime
+{
+    private NatsServerProcess _serverA = null!;
+    private NatsServerProcess _serverB = null!;
+
+    public int PortA => _serverA.Port;
+    public int PortB => _serverB.Port;
+
+    public async Task InitializeAsync()
+    {
+        var gwPortA = NatsServerProcess.AllocateFreePort();
+        var gwPortB = NatsServerProcess.AllocateFreePort();
+
+        var configA = $$"""
+            server_name: gw-a
+            gateway {
+                name: cluster-a
+                listen: 127.0.0.1:{{gwPortA}}
+                gateways: [
+                    { name: cluster-b, url: nats://127.0.0.1:{{gwPortB}} }
+                ]
+            }
+            """;
+
+        var configB = $$"""
+            server_name: gw-b
+            gateway {
+                name: cluster-b
+                listen: 127.0.0.1:{{gwPortB}}
+                gateways: [
+                    { name: cluster-a, url: nats://127.0.0.1:{{gwPortA}} }
+                ]
+            }
+            """;
+
+        _serverA = NatsServerProcess.WithConfig(configA, enableMonitoring: true);
+        _serverB = NatsServerProcess.WithConfig(configB, enableMonitoring: true);
+
+        await Task.WhenAll(_serverA.StartAsync(), _serverB.StartAsync());
+
+        // Poll until both gateways report a connected outbound gateway
+        await WaitForGatewayConnectionAsync();
+    }
+
+    private async Task WaitForGatewayConnectionAsync()
+    {
+        using var http = new HttpClient();
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(200));
+
+        var monitorPorts = new[] { _serverA.MonitorPort!.Value, _serverB.MonitorPort!.Value };
+
+        while (await timer.WaitForNextTickAsync(timeout.Token).ConfigureAwait(false))
+        {
+            var allConnected = true;
+            foreach (var monitorPort in monitorPorts)
+            {
+                try
+                {
+                    var gatewayz = await http.GetFromJsonAsync<Gatewayz>(
+                        $"http://127.0.0.1:{monitorPort}/gatewayz",
+                        timeout.Token);
+
+                    if (gatewayz?.NumGateways < 1)
+                    {
+                        allConnected = false;
+                        break;
+                    }
+                }
+                catch (HttpRequestException)
+                {
+                    // monitor not yet ready — retry on next tick
+                    allConnected = false;
+                    break;
+                }
+            }
+
+            if (allConnected)
+                return;
+        }
+
+        throw new TimeoutException("Gateways did not connect to each other within 30s.");
+    }
+
+    public async Task DisposeAsync()
+    {
+        await Task.WhenAll(
+            _serverA.DisposeAsync().AsTask(),
+            _serverB.DisposeAsync().AsTask());
+    }
+
+    public NatsConnection CreateClientA()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{PortA}" });
+
+    public NatsConnection CreateClientB()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{PortB}" });
+
+    private sealed class Gatewayz
+    {
+        [System.Text.Json.Serialization.JsonPropertyName("num_gateways")]
+        public int NumGateways { get; init; }
+    }
+}
+
+[CollectionDefinition("E2E-Gateway")]
+public class GatewayCollection : ICollectionFixture<GatewayFixture>;