Verify AirOS CPU count source

Follow-up to #62.

The AirOS backend currently derives resources.cpus from 1.3.6.1.4.1.10002.1.1.1.4, the load table. Before changing this behavior, verify the relevant AirOS MIBs on real devices and compare the reported value with the device CPU topology.

If this is not an authoritative CPU source, identify the correct supported OID or omit the field. Add a sanitized real-device fixture and regression test for the selected behavior.