Add CLI validation and DNS options to Connectivity Monitor

[Codex]

Summary

• add CLI switches for latency warning threshold, DNS enable/disable, and DNS target to the Python monitor
• validate config inputs (positive intervals, port range, required targets) and surface clear errors before running
• document the new options in python/README.md with examples

Testing

• python -m connectivity_monitor --help
• python - <<'PY'
  from argparse import Namespace
  from connectivity_monitor.config import headless_config, validate_config
  args = Namespace(targets="1.1.1.1,8.8.8.8", poll=5, threshold=3, lat_warn=80, enable_dns=False, dns_target="example.com", web_port=9000)
  cfg = validate_config(headless_config(args))
  print({k: cfg[k] for k in sorted(cfg) if k not in {"headless"}})
  bad_args = Namespace(targets="", poll=0, threshold=0, lat_warn=0, enable_dns=True, dns_target="", web_port=70000)
  try:
  validate_config(headless_config(bad_args))
  except ValueError as exc:
  print("Validation error:", exc)
  PY