refactor: extract NATS.Server.Transport.Tests project

Move TLS, OCSP, WebSocket, Networking, and IO test files from
NATS.Server.Tests into a dedicated NATS.Server.Transport.Tests
project. Update namespaces, replace private GetFreePort/ReadUntilAsync
with shared TestUtilities helpers, extract TestCertHelper to
TestUtilities, and replace Task.Delay polling loops with
PollHelper.WaitUntilAsync/YieldForAsync for proper synchronization.

tests/NATS.Server.Tests/MonitorTests.cs

@@ -5,6 +5,7 @@ using System.Net.Sockets;
 using System.Text;
 using Microsoft.Extensions.Logging.Abstractions;
 using NATS.Server.Monitoring;
+using NATS.Server.TestUtilities;
 
 namespace NATS.Server.Tests;
 
@@ -310,7 +311,7 @@ public class MonitorTlsTests : IAsyncLifetime
     {
         _natsPort = GetFreePort();
         _monitorPort = GetFreePort();
-        (_certPath, _keyPath) = TlsHelperTests.GenerateTestCertFiles();
+        (_certPath, _keyPath) = TestCertHelper.GenerateTestCertFiles();
         _server = new NatsServer(
             new NatsOptions
             {