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.

src/NATS.Server/NATS.Server.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <ItemGroup>
     <InternalsVisibleTo Include="NATS.Server.Tests" />
+    <InternalsVisibleTo Include="NATS.Server.Transport.Tests" />
   </ItemGroup>
   <ItemGroup>
     <FrameworkReference Include="Microsoft.AspNetCore.App" />