refactor: extract NATS.Server.JetStream.Tests project

Move 225 JetStream-related test files from NATS.Server.Tests into a
dedicated NATS.Server.JetStream.Tests project. This includes root-level
JetStream*.cs files, storage test files (FileStore, MemStore,
StreamStoreContract), and the full JetStream/ subfolder tree (Api,
Cluster, Consumers, MirrorSource, Snapshots, Storage, Streams).

Updated all namespaces, added InternalsVisibleTo, registered in the
solution file, and added the JETSTREAM_INTEGRATION_MATRIX define.

tests/NATS.Server.JetStream.Tests/JetStream/JetStreamCrossClusterRuntimeParityTests.cs

@@ -0,0 +1,26 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.Gateways;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamCrossClusterRuntimeParityTests
+{
+    [Fact]
+    public async Task Jetstream_cross_cluster_messages_are_forward_counted()
+    {
+        var manager = new GatewayManager(
+            new GatewayOptions { Host = "127.0.0.1", Port = 0, Name = "A" },
+            new ServerStats(),
+            "S1",
+            _ => { },
+            _ => { },
+            NullLogger<GatewayManager>.Instance);
+
+        await manager.ForwardJetStreamClusterMessageAsync(
+            new GatewayMessage("$JS.CLUSTER.REPL", null, "x"u8.ToArray()),
+            default);
+
+        manager.ForwardedJetStreamClusterMessages.ShouldBe(1);
+    }
+}