fix: reject invalid hosts payloads before publishing

[xuyua9]

Summary

• reject empty or incomplete DNS results before publishing generated hosts files
• reject duplicate or unknown hostnames, non-IPv4 mappings, and the DNS failure sentinel
• add regression coverage for the validation boundary

fetch_ips.py can continue after a DNS failure and append # IP Address Not Found to an otherwise publishable result. This change validates the complete mapping list in common.py before write_file or write_json_file runs, so a partial result cannot overwrite the generated artifacts.

Validation

• python3 -m unittest discover -s tests -p 'test_*.py' -v
• python3 -m py_compile common.py fetch_ips.py update_ips.py tests/test_hosts_payload_validation.py
• git diff --check