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/Monitoring/PprofEndpointTests.cs

@@ -0,0 +1,87 @@
+using System.Net;
+using System.Net.Sockets;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.Monitoring.Tests;
+
+public class PprofEndpointTests
+{
+    [Fact]
+    public async Task Debug_pprof_endpoint_returns_profile_index_when_profport_enabled()
+    {
+        await using var fx = await PprofMonitorFixture.StartWithProfilingAsync();
+        var body = await fx.GetStringAsync("/debug/pprof");
+        body.ShouldContain("profiles");
+    }
+}
+
+internal sealed class PprofMonitorFixture : IAsyncDisposable
+{
+    private readonly NatsServer _server;
+    private readonly CancellationTokenSource _cts;
+    private readonly HttpClient _http;
+    private readonly int _monitorPort;
+
+    private PprofMonitorFixture(NatsServer server, CancellationTokenSource cts, HttpClient http, int monitorPort)
+    {
+        _server = server;
+        _cts = cts;
+        _http = http;
+        _monitorPort = monitorPort;
+    }
+
+    public static async Task<PprofMonitorFixture> StartWithProfilingAsync()
+    {
+        var monitorPort = TestPortAllocator.GetFreePort();
+        var options = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            MonitorPort = monitorPort,
+            ProfPort = monitorPort,
+        };
+
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        var cts = new CancellationTokenSource();
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        var http = new HttpClient();
+        for (var i = 0; i < 50; i++)
+        {
+            try
+            {
+                var response = await http.GetAsync($"http://127.0.0.1:{monitorPort}/healthz");
+                if (response.IsSuccessStatusCode)
+                    break;
+            }
+            catch
+            {
+            }
+
+            await Task.Delay(50);
+        }
+
+        return new PprofMonitorFixture(server, cts, http, monitorPort);
+    }
+
+    public Task<string> GetStringAsync(string path)
+    {
+        return _http.GetStringAsync($"http://127.0.0.1:{_monitorPort}{path}");
+    }
+
+    public Task<byte[]> GetBytesAsync(string path)
+    {
+        return _http.GetByteArrayAsync($"http://127.0.0.1:{_monitorPort}{path}");
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        _http.Dispose();
+        await _cts.CancelAsync();
+        _server.Dispose();
+        _cts.Dispose();
+    }
+
+}