test: add E2E monitoring endpoint tests (varz, connz, healthz)

Replace Task.Delay polling with PeriodicTimer in NatsServerProcess readiness
checks and extend StartAsync to also TCP-poll the monitor port when enabled,
so MonitorServerFixture is guaranteed ready before tests run.

tests/NATS.E2E.Tests/Infrastructure/NatsServerProcess.cs

@@ -100,6 +100,9 @@ public sealed class NatsServerProcess : IAsyncDisposable
         _process.BeginErrorReadLine();
 
         await WaitForTcpReadyAsync();
+
+        if (_enableMonitoring && MonitorPort.HasValue)
+            await WaitForMonitorPortReadyAsync();
     }
 
     public async ValueTask DisposeAsync()
@@ -115,9 +118,11 @@ public sealed class NatsServerProcess : IAsyncDisposable
                 {
                     await _process.WaitForExitAsync(cts.Token);
                 }
-                catch (OperationCanceledException)
+                catch (OperationCanceledException ex) when (!_process.HasExited)
                 {
-                    // Already killed the tree above; nothing more to do
+                    // Kill timed out and process is still running — force-terminate and surface the error
+                    throw new InvalidOperationException(
+                        $"NATS server process did not exit within 5s after kill.", ex);
                 }
             }
 
@@ -136,8 +141,10 @@ public sealed class NatsServerProcess : IAsyncDisposable
     private async Task WaitForTcpReadyAsync()
     {
         using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(100));
+        SocketException? lastError = null;
 
-        while (!timeout.Token.IsCancellationRequested)
+        while (await timer.WaitForNextTickAsync(timeout.Token).ConfigureAwait(false))
         {
             try
             {
@@ -145,14 +152,38 @@ public sealed class NatsServerProcess : IAsyncDisposable
                 await socket.ConnectAsync(new IPEndPoint(IPAddress.Loopback, Port), timeout.Token);
                 return; // Connected — server is ready
             }
-            catch (SocketException)
+            catch (SocketException ex)
             {
-                await Task.Delay(100, timeout.Token);
+                lastError = ex; // Server not yet accepting connections — retry on next tick
             }
         }
 
         throw new TimeoutException(
-            $"NATS server did not become ready on port {Port} within 10s.\n\nServer output:\n{Output}");
+            $"NATS server did not become ready on port {Port} within 10s. Last error: {lastError?.Message}\n\nServer output:\n{Output}");
+    }
+
+    private async Task WaitForMonitorPortReadyAsync()
+    {
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        using var timer = new PeriodicTimer(TimeSpan.FromMilliseconds(100));
+        SocketException? lastError = null;
+
+        while (await timer.WaitForNextTickAsync(timeout.Token).ConfigureAwait(false))
+        {
+            try
+            {
+                using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+                await socket.ConnectAsync(new IPEndPoint(IPAddress.Loopback, MonitorPort!.Value), timeout.Token);
+                return; // Monitor HTTP port is accepting connections
+            }
+            catch (SocketException ex)
+            {
+                lastError = ex; // Monitor not yet accepting connections — retry on next tick
+            }
+        }
+
+        throw new TimeoutException(
+            $"NATS monitor port {MonitorPort} did not become ready within 10s. Last error: {lastError?.Message}\n\nServer output:\n{Output}");
     }
 
     private static string ResolveHostDll()