refactor: extract NATS.Server.Clustering.Tests project

Move 29 clustering/routing test files from NATS.Server.Tests to a
dedicated NATS.Server.Clustering.Tests project. Update namespaces,
replace private GetFreePort/ReadUntilAsync helpers with TestUtilities
calls, and extract TestServerFactory/ClusterTestServer to TestUtilities
to fix cross-project reference from JetStreamStartupTests.

tests/NATS.Server.Clustering.Tests/Routes/RouteTopologyGossipTests.cs

@@ -0,0 +1,25 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.Routes;
+
+namespace NATS.Server.Clustering.Tests;
+
+public class RouteTopologyGossipTests
+{
+    [Fact]
+    public void Topology_snapshot_reports_server_and_route_counts()
+    {
+        var manager = new RouteManager(
+            new ClusterOptions { Host = "127.0.0.1", Port = 0 },
+            new ServerStats(),
+            "S1",
+            _ => { },
+            _ => { },
+            NullLogger<RouteManager>.Instance);
+
+        var snapshot = manager.BuildTopologySnapshot();
+        snapshot.ServerId.ShouldBe("S1");
+        snapshot.RouteCount.ShouldBe(0);
+        snapshot.ConnectedServerIds.ShouldBeEmpty();
+    }
+}