feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/JetStreamInternalClientTests.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+
+namespace NATS.Server.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");
+    }
+}