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/JetStreamInternalClientTests.cs

@@ -0,0 +1,38 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.JetStream;
+using System.Reflection;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamInternalClientTests
+{
+    [Fact]
+    public async Task JetStream_enabled_server_creates_internal_jetstream_client_and_keeps_it_account_scoped()
+    {
+        var options = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            JetStream = new JetStreamOptions
+            {
+                StoreDir = Path.Combine(Path.GetTempPath(), $"nats-js-internal-{Guid.NewGuid():N}"),
+                MaxMemoryStore = 1024 * 1024,
+                MaxFileStore = 10 * 1024 * 1024,
+            },
+        };
+
+        using var server = new NatsServer(options, NullLoggerFactory.Instance);
+        using var cts = new CancellationTokenSource();
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        server.JetStreamInternalClient.ShouldNotBeNull();
+        server.JetStreamInternalClient!.Kind.ShouldBe(ClientKind.JetStream);
+        server.JetStreamInternalClient.Account?.Name.ShouldBe("$SYS");
+
+        var serviceField = typeof(NatsServer).GetField("_jetStreamService", BindingFlags.Instance | BindingFlags.NonPublic);
+        serviceField.ShouldNotBeNull();
+        (serviceField!.GetValue(server) as JetStreamService).ShouldNotBeNull();
+    }
+}