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/RouteRmsgForwardingTests.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Clustering.Tests;
+
+public class RouteRmsgForwardingTests
+{
+    [Fact]
+    public async Task Publish_on_serverA_reaches_remote_subscriber_on_serverB_via_RMSG()
+    {
+        await using var fx = await RouteFixture.StartTwoNodeClusterAsync();
+        await fx.SubscribeOnServerBAsync("foo.>");
+
+        await fx.PublishFromServerAAsync("foo.bar", "payload");
+        (await fx.ReadServerBMessageAsync()).ShouldContain("payload");
+    }
+}