Add backward compatibility for PING results from older agents

Older agents send PING job results as SnmpResult messages instead of
MonitoringCheck messages, causing FunctionClauseError when
process_job_result/3 tried to pattern match on job_type: :PING.

Added handler that:
- Catches PING results in SnmpResult format
- Logs warning that agent should be upgraded
- Creates monitoring check with success status
- Stores result via store_monitoring_check

This prevents crashes while maintaining backward compatibility until
all production agents are upgraded to use MonitoringCheck format.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

lib/towerops_web/channels/agent_channel.ex

@@ -1163,6 +1163,28 @@ defmodule ToweropsWeb.AgentChannel do
     process_polling_result(device, result, socket)
   end
 
+  defp process_job_result(device, %{job_type: :PING} = result, socket) do
+    # Backward compatibility: older agents send PING results as SnmpResult instead of MonitoringCheck
+    # Newer agents should use the "monitoring_check" event instead
+    Logger.warning("Agent sent PING result as SnmpResult (deprecated). Agent should be upgraded.",
+      agent_token_id: socket.assigns.agent_token_id,
+      device_id: device.id,
+      job_id: result.job_id
+    )
+
+    # PING jobs sent as SnmpResult have empty oid_values
+    # If we receive a result (not an error), assume success
+    check = %{
+      device_id: device.id,
+      status: "success",
+      # Unknown - older agents don't send this
+      response_time_ms: 0.0,
+      timestamp: result.timestamp
+    }
+
+    store_monitoring_check(check, socket)
+  end
+
   defp process_discovery_result(device, result, socket) do
     Logger.info("Discovery results received from agent for #{device.name}, processing data")