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/RouteAccountScopedTests.cs

@@ -0,0 +1,14 @@
+using NATS.Server.Routes;
+
+namespace NATS.Server.Clustering.Tests;
+
+public class RouteAccountScopedTests
+{
+    [Fact]
+    public void Route_connect_info_includes_account_scope()
+    {
+        var json = RouteConnection.BuildConnectInfoJson("S1", ["A"], "topology-v1");
+        json.ShouldContain("\"accounts\":[\"A\"]");
+        json.ShouldContain("\"topology\":\"topology-v1\"");
+    }
+}