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.Tests/JetStreamStartupTests.cs

@@ -1,3 +1,4 @@
+using NATS.Server.TestUtilities;
 namespace NATS.Server.Tests;
 
 public class JetStreamStartupTests