feat: phase B distributed substrate test parity — 39 new tests across 5 subsystems

FileStore basics (4), MemStore/retention (10), RAFT election/append (16),
config reload parity (3), monitoring endpoints varz/connz/healthz (6).
972 total tests passing, 0 failures.

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

@@ -0,0 +1,82 @@
+// Ported from golang/nats-server/server/monitor_test.go
+// TestMonitorHealthzStatusOK — verify /healthz returns HTTP 200 with status "ok".
+
+using System.Net;
+using System.Net.Sockets;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace NATS.Server.Tests;
+
+public class HealthzParityTests : IAsyncLifetime
+{
+    private readonly NatsServer _server;
+    private readonly int _monitorPort;
+    private readonly CancellationTokenSource _cts = new();
+    private readonly HttpClient _http = new();
+
+    public HealthzParityTests()
+    {
+        _monitorPort = GetFreePort();
+        _server = new NatsServer(
+            new NatsOptions { Port = 0, MonitorPort = _monitorPort },
+            NullLoggerFactory.Instance);
+    }
+
+    public async Task InitializeAsync()
+    {
+        _ = _server.StartAsync(_cts.Token);
+        await _server.WaitForReadyAsync();
+        for (var i = 0; i < 50; i++)
+        {
+            try
+            {
+                var probe = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+                if (probe.IsSuccessStatusCode) break;
+            }
+            catch (HttpRequestException) { }
+            await Task.Delay(50);
+        }
+    }
+
+    public async Task DisposeAsync()
+    {
+        _http.Dispose();
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+    /// <summary>
+    /// Corresponds to Go TestMonitorHealthzStatusOK.
+    /// Verifies GET /healthz returns HTTP 200 OK, indicating the server is healthy.
+    /// </summary>
+    [Fact]
+    public async Task Healthz_returns_ok()
+    {
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+    }
+
+    /// <summary>
+    /// Corresponds to Go TestMonitorHealthzStatusOK / checkHealthStatus.
+    /// Verifies the /healthz response body contains the "ok" status string,
+    /// matching the Go server's HealthStatus.Status = "ok" field.
+    /// </summary>
+    [Fact]
+    public async Task Healthz_returns_status_ok_json()
+    {
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var body = await response.Content.ReadAsStringAsync();
+        // The .NET monitoring server returns Results.Ok("ok") which serializes as the JSON string "ok".
+        // This corresponds to the Go server's HealthStatus.Status = "ok".
+        body.ShouldContain("ok");
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+}