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.

lib/towerops_web/channels/agent_channel.ex

@@ -456,14 +456,14 @@ defmodule ToweropsWeb.AgentChannel do
          {:ok, error} <- Validator.validate_agent_error(binary) do
       maybe_debug_log(socket, "Agent job error",
         device_id: error.device_id,
-        error_message: error.error_message,
+        error_message: error.message,
         binary_size: byte_size(binary_b64)
       )
 
       Logger.error("Agent job error",
         agent_token_id: socket.assigns.agent_token_id,
         device_id: error.device_id,
-        error: error.error_message
+        error: error.message
       )
 
       {:noreply, socket}