fix: split net_socket_fd test for Windows compatibility
The Unix socket test uses .sock paths which don't work on Windows.
Split into two tests:
- net_socket_fd_pipe (unix-only): exercises net.Socket({ fd }) via
Unix domain sockets
- net_socket_fd_invalid (cross-platform): verifies ERR_INVALID_FD_TYPE
for non-pipe/tcp file descriptors
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
