Add IP address table walks to agent discovery queries

Fixes missing IP addresses tab on device page after agent-based discovery.

Issue:
Discovery was not collecting IP address data because the agent's
discovery job didn't include IP address table OID walks.

Root cause:
build_discovery_queries() walked interface tables, sensors, vendor MIBs,
and neighbors, but was missing the IP-MIB address tables.

Changes:
- Add new SnmpQuery to walk IP address tables during discovery:
  - 1.3.6.1.2.1.4.20 (ipAddrTable - IPv4, deprecated but widely supported)
  - 1.3.6.1.2.1.4.34 (ipAddressTable - RFC 4293, unified IPv4/IPv6)

This matches the OIDs used by Phoenix's direct discovery in
lib/towerops/snmp/profiles/base.ex discover_all_ip_addresses/1.

Result:
Agent now collects IP address data during discovery, which is then
synced to the database and displayed in the IP Addresses tab.

Tests: 5578 tests passing (1 pre-existing failure in unrelated Dahua test)

lib/towerops_web/channels/agent_channel.ex

@@ -674,6 +674,16 @@ defmodule ToweropsWeb.AgentChannel do
           # Cisco CDP
           "1.3.6.1.4.1.9.9.23"
         ]
+      },
+      # IP address discovery (WALK)
+      %SnmpQuery{
+        query_type: :WALK,
+        oids: [
+          # IP-MIB ipAddrTable (IPv4 - deprecated but widely supported)
+          "1.3.6.1.2.1.4.20",
+          # IP-MIB ipAddressTable (RFC 4293 - unified IPv4/IPv6)
+          "1.3.6.1.2.1.4.34"
+        ]
       }
     ]
   end