Move 29 MQTT test files from NATS.Server.Tests into a dedicated
NATS.Server.Mqtt.Tests project. Update namespaces, add
InternalsVisibleTo, and replace Task.Delay calls with
PollHelper.WaitUntilAsync for proper synchronization.
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.
Add shared test utility library with TestPortAllocator.GetFreePort() and
SocketTestHelper.ReadUntilAsync() to deduplicate helpers across test projects.
This is the foundation for splitting the monolithic test project into
feature-focused test projects.
