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.TestUtilities/SocketTestHelper.cs

@@ -18,4 +18,18 @@ public static class SocketTestHelper
         }
         return sb.ToString();
     }
+
+    public static async Task<string> ReadUntilAsync(Stream stream, string expected, int timeoutMs = 5000)
+    {
+        using var cts = new CancellationTokenSource(timeoutMs);
+        var sb = new StringBuilder();
+        var buf = new byte[4096];
+        while (!sb.ToString().Contains(expected, StringComparison.Ordinal))
+        {
+            var n = await stream.ReadAsync(buf, cts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        return sb.ToString();
+    }
 }

tests/NATS.Server.TestUtilities/TestCertHelper.cs

@@ -0,0 +1,31 @@
+using System.Net;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+
+namespace NATS.Server.TestUtilities;
+
+public static class TestCertHelper
+{
+    public static (string certPath, string keyPath) GenerateTestCertFiles()
+    {
+        var (cert, key) = GenerateTestCert();
+        var certPath = Path.GetTempFileName();
+        var keyPath = Path.GetTempFileName();
+        File.WriteAllText(certPath, cert.ExportCertificatePem());
+        File.WriteAllText(keyPath, key.ExportPkcs8PrivateKeyPem());
+        return (certPath, keyPath);
+    }
+
+    public static (X509Certificate2 cert, RSA key) GenerateTestCert()
+    {
+        var key = RSA.Create(2048);
+        var req = new CertificateRequest("CN=localhost", key, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        req.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, false));
+        var sanBuilder = new SubjectAlternativeNameBuilder();
+        sanBuilder.AddIpAddress(IPAddress.Loopback);
+        sanBuilder.AddDnsName("localhost");
+        req.CertificateExtensions.Add(sanBuilder.Build());
+        var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow, DateTimeOffset.UtcNow.AddYears(1));
+        return (cert, key);
+    }
+}