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/RouteCompressionTests.cs

@@ -0,0 +1,16 @@
+using System.Text;
+using NATS.Server.Routes;
+
+namespace NATS.Server.Clustering.Tests;
+
+public class RouteCompressionTests
+{
+    [Fact]
+    public void Route_payload_round_trips_through_compression_codec()
+    {
+        var payload = Encoding.UTF8.GetBytes(new string('x', 512));
+        var compressed = RouteCompressionCodec.Compress(payload);
+        var restored = RouteCompressionCodec.Decompress(compressed);
+        restored.ShouldBe(payload);
+    }
+}