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.
Validates the server against the official NATS .NET client library with
tests for basic pub/sub, wildcard (* and >) matching, fan-out to
multiple subscribers, and PING/PONG keepalive. All 5 tests pass without
requiring any server changes.