getSubnet wraps its record under `subnet4`. The driver read `subnet`, got
nothing, and carried on:
- get_dhcp_subnets() reported every subnet with no pools, no options and no
description. Only the CIDR survived, and only because it falls back to the
searchSubnet row. Confirmed against a live OPNsense serving six subnets:
all six came back with empty pools while the device had
"10.10.0.100-10.10.0.250" and routers/DNS/NTP set on each.
- apply_dhcp_subnet() read the same key to merge the options it was not
asked to change. An empty record means nothing to preserve, so updating a
subnet with only domain_search set would have written back only that one
option and blanked the routers Kea autocollected — stranding every client
on that VLAN without a gateway. That is precisely the failure the merge
exists to prevent.
The unit fixtures encoded the wrong shape, which is why the safety test
test_unnamed_options_are_preserved_on_update passed while the real thing was
broken. They now carry the response captured from OPNsense 25.x, and correcting
them turns that test red against the old parse.
Both call sites go through _kea_subnet_record(), which prefers `subnet4` and
falls back to `subnet` for older builds.