feat: add cluster config hot reload (Gap 14.4)

Adds ClusterConfigChangeResult and ApplyClusterConfigChanges to ConfigReloader,
comparing route/gateway/leaf URL sets between old and new NatsOptions and reporting
added/removed routes for connection reconciliation on hot reload.

tests/NATS.Server.Tests/Configuration/ClusterConfigReloadTests.cs

@@ -0,0 +1,186 @@
+// Tests for ConfigReloader.ApplyClusterConfigChanges.
+// Go reference: golang/nats-server/server/reload.go — routesOption.Apply, gatewayOption.Apply.
+
+using NATS.Server;
+using NATS.Server.Configuration;
+using Shouldly;
+
+namespace NATS.Server.Tests.Configuration;
+
+public class ClusterConfigReloadTests
+{
+    // ─── helpers ────────────────────────────────────────────────────
+
+    private static NatsOptions WithRoutes(params string[] routes)
+    {
+        var opts = new NatsOptions();
+        opts.Cluster = new ClusterOptions { Routes = [..routes] };
+        return opts;
+    }
+
+    private static NatsOptions WithGatewayRemotes(params string[] urls)
+    {
+        var opts = new NatsOptions();
+        opts.Gateway = new GatewayOptions
+        {
+            RemoteGateways = [new RemoteGatewayOptions { Urls = [..urls] }]
+        };
+        return opts;
+    }
+
+    private static NatsOptions WithLeafRemotes(params string[] urls)
+    {
+        var opts = new NatsOptions();
+        opts.LeafNode = new LeafNodeOptions { Remotes = [..urls] };
+        return opts;
+    }
+
+    // ─── tests ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void No_changes_returns_no_changes()
+    {
+        // Go reference: reload.go routesOption.Apply — no-op when sets are equal
+        var old = WithRoutes("nats://server1:6222", "nats://server2:6222");
+        var updated = WithRoutes("nats://server1:6222", "nats://server2:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeFalse();
+        result.RouteUrlsChanged.ShouldBeFalse();
+        result.GatewayUrlsChanged.ShouldBeFalse();
+        result.LeafUrlsChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Route_url_added_detected()
+    {
+        // Go reference: reload.go routesOption.Apply — new route triggers reconnect
+        var old = WithRoutes("nats://server1:6222");
+        var updated = WithRoutes("nats://server1:6222", "nats://server2:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.RouteUrlsChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Route_url_removed_detected()
+    {
+        // Go reference: reload.go routesOption.Apply — removed route triggers disconnect
+        var old = WithRoutes("nats://server1:6222", "nats://server2:6222");
+        var updated = WithRoutes("nats://server1:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.RouteUrlsChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Gateway_url_changed_detected()
+    {
+        // Go reference: reload.go gatewayOption.Apply — gateway remotes reconciled on reload
+        var old = WithGatewayRemotes("nats://gw1:7222");
+        var updated = WithGatewayRemotes("nats://gw2:7222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.GatewayUrlsChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Leaf_url_changed_detected()
+    {
+        // Go reference: reload.go leafNodeOption.Apply — leaf remotes reconciled on reload
+        var old = WithLeafRemotes("nats://hub:5222");
+        var updated = WithLeafRemotes("nats://hub2:5222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.LeafUrlsChanged.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Multiple_changes_detected()
+    {
+        // Go reference: reload.go — multiple topology changes in a single reload
+        var old = new NatsOptions
+        {
+            Cluster = new ClusterOptions { Routes = ["nats://r1:6222"] },
+            Gateway = new GatewayOptions { RemoteGateways = [new RemoteGatewayOptions { Urls = ["nats://gw1:7222"] }] }
+        };
+        var updated = new NatsOptions
+        {
+            Cluster = new ClusterOptions { Routes = ["nats://r1:6222", "nats://r2:6222"] },
+            Gateway = new GatewayOptions { RemoteGateways = [new RemoteGatewayOptions { Urls = ["nats://gw2:7222"] }] }
+        };
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.RouteUrlsChanged.ShouldBeTrue();
+        result.GatewayUrlsChanged.ShouldBeTrue();
+        result.LeafUrlsChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Same_urls_different_order_no_change()
+    {
+        // Go reference: reload.go — order-independent URL comparison
+        var old = WithRoutes("nats://server1:6222", "nats://server2:6222");
+        var updated = WithRoutes("nats://server2:6222", "nats://server1:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeFalse();
+        result.RouteUrlsChanged.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void AddedRouteUrls_lists_new_routes()
+    {
+        // Go reference: reload.go routesOption.Apply — identifies routes to dial
+        var old = WithRoutes("nats://server1:6222");
+        var updated = WithRoutes("nats://server1:6222", "nats://server2:6222", "nats://server3:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.AddedRouteUrls.Count.ShouldBe(2);
+        result.AddedRouteUrls.ShouldContain("nats://server2:6222");
+        result.AddedRouteUrls.ShouldContain("nats://server3:6222");
+        result.RemovedRouteUrls.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void RemovedRouteUrls_lists_removed_routes()
+    {
+        // Go reference: reload.go routesOption.Apply — identifies routes to close
+        var old = WithRoutes("nats://server1:6222", "nats://server2:6222", "nats://server3:6222");
+        var updated = WithRoutes("nats://server1:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.RemovedRouteUrls.Count.ShouldBe(2);
+        result.RemovedRouteUrls.ShouldContain("nats://server2:6222");
+        result.RemovedRouteUrls.ShouldContain("nats://server3:6222");
+        result.AddedRouteUrls.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void Empty_to_non_empty_detected()
+    {
+        // Go reference: reload.go routesOption.Apply — nil→populated triggers dial
+        var old = new NatsOptions();   // no Cluster configured
+        var updated = WithRoutes("nats://server1:6222");
+
+        var result = ConfigReloader.ApplyClusterConfigChanges(old, updated);
+
+        result.HasChanges.ShouldBeTrue();
+        result.RouteUrlsChanged.ShouldBeTrue();
+        result.AddedRouteUrls.ShouldContain("nats://server1:6222");
+    }
+}