feat(cluster): add implicit route and gateway discovery via INFO gossip

Implements ProcessImplicitRoute and ForwardNewRouteInfoToKnownServers on RouteManager,
and ProcessImplicitGateway on GatewayManager, mirroring Go server/route.go and
server/gateway.go INFO gossip-based peer discovery. Adds ConnectUrls to ServerInfo
and introduces GatewayInfo model. 12 new unit tests in ImplicitDiscoveryTests.

src/NATS.Server/Gateways/GatewayInfo.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Gateways;
+
+/// <summary>
+/// Information about a remote gateway cluster received during implicit discovery.
+/// Go reference: server/gateway.go — implicit gateway discovery via INFO gossip.
+/// </summary>
+public sealed record GatewayInfo
+{
+    /// <summary>Name of the remote gateway cluster.</summary>
+    public required string Name { get; init; }
+
+    /// <summary>URLs for connecting to the remote gateway cluster.</summary>
+    public required string[] Urls { get; init; }
+}

src/NATS.Server/Gateways/GatewayManager.cs

@@ -16,6 +16,7 @@ public sealed class GatewayManager : IAsyncDisposable
     private readonly Action<GatewayMessage> _messageSink;
     private readonly ILogger<GatewayManager> _logger;
     private readonly ConcurrentDictionary<string, GatewayConnection> _connections = new(StringComparer.Ordinal);
+    private readonly HashSet<string> _discoveredGateways = new(StringComparer.OrdinalIgnoreCase);
     private long _forwardedJetStreamClusterMessages;
 
     private CancellationTokenSource? _cts;
@@ -44,6 +45,29 @@ public sealed class GatewayManager : IAsyncDisposable
         _logger = logger;
     }
 
+    /// <summary>
+    /// Gateway clusters auto-discovered via INFO gossip.
+    /// Go reference: server/gateway.go processImplicitGateway.
+    /// </summary>
+    public IReadOnlyCollection<string> DiscoveredGateways
+    {
+        get { lock (_discoveredGateways) return _discoveredGateways.ToList(); }
+    }
+
+    /// <summary>
+    /// Processes a gateway info message from a peer, discovering new gateway clusters.
+    /// Go reference: server/gateway.go:800-850 (processImplicitGateway).
+    /// </summary>
+    public void ProcessImplicitGateway(GatewayInfo gwInfo)
+    {
+        ArgumentNullException.ThrowIfNull(gwInfo);
+
+        lock (_discoveredGateways)
+        {
+            _discoveredGateways.Add(gwInfo.Name);
+        }
+    }
+
     public Task StartAsync(CancellationToken ct)
     {
         _cts = CancellationTokenSource.CreateLinkedTokenSource(ct);