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/MsgTraceGoParityTests.cs

@@ -15,6 +15,7 @@ using NATS.Server;
 using NATS.Server.Auth;
 using NATS.Server.Monitoring;
 using NATS.Server.Protocol;
+using NATS.Server.TestUtilities;
 
 namespace NATS.Server.Tests;
 
@@ -784,7 +785,7 @@ public class MsgTraceGoParityTests : IAsyncLifetime
     public async Task ClosedConns_tls_handshake_close_reason_tracked()
     {
         // Go: TestClosedTLSHandshake (closed_conns_test.go:247)
-        var (certPath, keyPath) = TlsHelperTests.GenerateTestCertFiles();
+        var (certPath, keyPath) = TestCertHelper.GenerateTestCertFiles();
         try
         {
             var port = GetFreePort();