Make get_mac_from_arp take an arp_path for testability (default unchanged), and
add tests for get_client_ip (X-Forwarded-For vs remote_addr), ARP MAC
resolution (match/no-match/missing), SAIP decode-unavailable, and the
SAIP_ASN_PATH override. Coverage: utils 100%, overall 93%; 108 tests pass.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>