fix: address MonitorServer review — dispose resources, add cancellation, improve test reliability

tests/NATS.Server.Tests/MonitorTests.cs

@@ -27,8 +27,17 @@ public class MonitorTests : IAsyncLifetime
     {
         _ = _server.StartAsync(_cts.Token);
         await _server.WaitForReadyAsync();
-        // Give monitoring server time to start
-        await Task.Delay(200);
+        // Wait for monitoring HTTP server to be ready
+        for (int i = 0; i < 50; i++)
+        {
+            try
+            {
+                var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/healthz");
+                if (response.IsSuccessStatusCode) break;
+            }
+            catch (HttpRequestException) { }
+            await Task.Delay(50);
+        }
     }
 
     public async Task DisposeAsync()