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/MonitorServerFixture.cs

@@ -16,7 +16,7 @@ public sealed class MonitorServerFixture : IAsyncLifetime
     public async Task InitializeAsync()
     {
         _server = new NatsServerProcess(enableMonitoring: true);
-        await _server.StartAsync();
+        await _server.StartAsync(); // StartAsync polls both the NATS port and the monitor TCP port before returning
         MonitorClient = new HttpClient { BaseAddress = new Uri($"http://127.0.0.1:{MonitorPort}") };
     }