refactor: extract NATS.Server.Monitoring.Tests project

Move 39 monitoring, events, and system endpoint test files from
NATS.Server.Tests into a dedicated NATS.Server.Monitoring.Tests project.
Update namespaces, replace private GetFreePort/ReadUntilAsync with
TestUtilities shared helpers, add InternalsVisibleTo, and register
in the solution file. All 439 tests pass.

tests/NATS.Server.Monitoring.Tests/JszMonitorTests.cs

@@ -0,0 +1,107 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Net.Sockets;
+using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.Monitoring;
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.Monitoring.Tests;
+
+public class JszMonitorTests
+{
+    [Fact]
+    public async Task Jsz_reports_live_stream_and_consumer_counts()
+    {
+        await using var fixture = await JetStreamMonitoringFixture.StartWithStreamAndConsumerAsync();
+
+        var jsz = await fixture.GetJszAsync();
+        jsz.Streams.ShouldBeGreaterThan(0);
+        jsz.Consumers.ShouldBeGreaterThan(0);
+    }
+}
+
+internal sealed class JetStreamMonitoringFixture : IAsyncDisposable
+{
+    private readonly NatsServer _server;
+    private readonly int _monitorPort;
+    private readonly CancellationTokenSource _cts = new();
+    private readonly HttpClient _http = new();
+
+    private JetStreamMonitoringFixture(NatsServer server, int monitorPort)
+    {
+        _server = server;
+        _monitorPort = monitorPort;
+    }
+
+    public static async Task<JetStreamMonitoringFixture> StartWithStreamAndConsumerAsync()
+    {
+        var natsPort = TestPortAllocator.GetFreePort();
+        var monitorPort = TestPortAllocator.GetFreePort();
+        var options = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = natsPort,
+            MonitorHost = "127.0.0.1",
+            MonitorPort = monitorPort,
+            JetStream = new JetStreamOptions
+            {
+                StoreDir = Path.Combine(Path.GetTempPath(), "natsdotnet-jsz"),
+                MaxMemoryStore = 1_024 * 1_024,
+                MaxFileStore = 10 * 1_024 * 1_024,
+            },
+        };
+
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        var fixture = new JetStreamMonitoringFixture(server, monitorPort);
+
+        _ = server.StartAsync(fixture._cts.Token);
+        await server.WaitForReadyAsync();
+        await fixture.WaitForHealthAsync();
+
+        var router = server.JetStreamApiRouter ?? throw new InvalidOperationException("JetStream API router unavailable.");
+        _ = router.Route("$JS.API.STREAM.CREATE.ORDERS", Encoding.UTF8.GetBytes("{\"name\":\"ORDERS\",\"subjects\":[\"orders.*\"]}"));
+        _ = router.Route("$JS.API.CONSUMER.CREATE.ORDERS.DUR", Encoding.UTF8.GetBytes("{\"durable_name\":\"DUR\",\"filter_subject\":\"orders.*\"}"));
+
+        return fixture;
+    }
+
+    public async Task<JszResponse> GetJszAsync()
+    {
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/jsz");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var jsz = await response.Content.ReadFromJsonAsync<JszResponse>();
+        return jsz ?? throw new InvalidOperationException("Failed to deserialize /jsz.");
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        _http.Dispose();
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+    private async Task WaitForHealthAsync()
+    {
+        for (int i = 0; i < 50; i++)
+        {
+            try
+            {
+                var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+                if (response.IsSuccessStatusCode)
+                    return;
+            }
+            catch (HttpRequestException)
+            {
+                // server not ready
+            }
+
+            await Task.Delay(50);
+        }
+
+        throw new TimeoutException("Monitoring endpoint did not become healthy.");
+    }
+
+}