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/RouteHandshakeTests.cs

@@ -0,0 +1,25 @@
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.Clustering.Tests;
+
+public class RouteHandshakeTests
+{
+    [Fact]
+    public async Task Two_servers_establish_route_connection()
+    {
+        await using var a = await TestServerFactory.CreateClusterEnabledAsync();
+        await using var b = await TestServerFactory.CreateClusterEnabledAsync(seed: a.ClusterListen);
+
+        await a.WaitForReadyAsync();
+        await b.WaitForReadyAsync();
+
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        while (!timeout.IsCancellationRequested && (a.Stats.Routes == 0 || b.Stats.Routes == 0))
+        {
+            await Task.Delay(50, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
+        }
+
+        a.Stats.Routes.ShouldBeGreaterThan(0);
+        b.Stats.Routes.ShouldBeGreaterThan(0);
+    }
+}