test: expand agent channel coverage from 8% to 66%
add 54 tests covering join/terminate lifecycle, heartbeat, error,
credential_test_result, monitoring_check, mikrotik_result, catch-all
handle_in events, handle_info lifecycle messages, PubSub messages,
SNMP result edge cases, poll result sensor/interface data storage,
SNMPv3 credential paths, and monitoring-enabled device job building.
fix bug where error.error_message was used instead of error.message
matching the AgentError protobuf struct field name.
Graham McIntire
f00dc3ff12