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);

src/NATS.Server/Protocol/NatsProtocol.cs

@@ -89,6 +89,10 @@ public sealed class ServerInfo
     [JsonPropertyName("tls_available")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
     public bool TlsAvailable { get; set; }
+
+    [JsonPropertyName("connect_urls")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string[]? ConnectUrls { get; set; }
 }
 
 public sealed class ClientOptions

src/NATS.Server/Routes/RouteManager.cs

@@ -3,6 +3,7 @@ using System.Net;
 using System.Net.Sockets;
 using Microsoft.Extensions.Logging;
 using NATS.Server.Configuration;
+using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
 
 namespace NATS.Server.Routes;
@@ -18,6 +19,8 @@ public sealed class RouteManager : IAsyncDisposable
     private readonly Action<RouteMessage> _routedMessageSink;
     private readonly ConcurrentDictionary<string, RouteConnection> _routes = new(StringComparer.Ordinal);
     private readonly ConcurrentDictionary<string, byte> _connectedServerIds = new(StringComparer.Ordinal);
+    private readonly HashSet<string> _discoveredRoutes = new(StringComparer.OrdinalIgnoreCase);
+    private readonly HashSet<string> _knownRouteUrls = new(StringComparer.OrdinalIgnoreCase);
 
     private CancellationTokenSource? _cts;
     private Socket? _listener;
@@ -49,6 +52,62 @@ public sealed class RouteManager : IAsyncDisposable
         _logger = logger;
     }
 
+    /// <summary>
+    /// Routes auto-discovered via INFO gossip from peers.
+    /// Go reference: server/route.go processImplicitRoute.
+    /// </summary>
+    public IReadOnlyCollection<string> DiscoveredRoutes
+    {
+        get { lock (_discoveredRoutes) return _discoveredRoutes.ToList(); }
+    }
+
+    /// <summary>
+    /// Event raised when new route info should be forwarded to known peers.
+    /// Go reference: server/route.go forwardNewRouteInfoToKnownServers.
+    /// </summary>
+    public event Action<List<string>>? OnForwardInfo;
+
+    /// <summary>
+    /// Processes connect_urls from a peer's INFO message. Any URLs not already
+    /// known are added to DiscoveredRoutes for solicited connection.
+    /// Go reference: server/route.go:1500-1550 (processImplicitRoute).
+    /// </summary>
+    public void ProcessImplicitRoute(ServerInfo serverInfo)
+    {
+        if (serverInfo.ConnectUrls is null || serverInfo.ConnectUrls.Length == 0)
+            return;
+
+        lock (_discoveredRoutes)
+        {
+            foreach (var url in serverInfo.ConnectUrls)
+            {
+                if (!_knownRouteUrls.Contains(url))
+                {
+                    _discoveredRoutes.Add(url);
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Forwards new peer URL information to all known route connections.
+    /// Go reference: server/route.go forwardNewRouteInfoToKnownServers.
+    /// </summary>
+    public void ForwardNewRouteInfoToKnownServers(string newPeerUrl)
+    {
+        OnForwardInfo?.Invoke([newPeerUrl]);
+    }
+
+    /// <summary>
+    /// Adds a URL to the known route set. Used during initialization and testing.
+    /// </summary>
+    public void AddKnownRoute(string url)
+    {
+        lock (_discoveredRoutes)
+        {
+            _knownRouteUrls.Add(url);
+        }
+    }
 
     /// <summary>
     /// Returns a route pool index for the given account name, matching Go's