fix: populate per-network HCG for vrouters after 2026.1 upgrade

[skrobul]

After upgrading to neutron 28 (2026.1), attaching a subnet to a router whose external gateway is on a vxlan-type network leaves the per-network unified HA_Chassis_Group (neutron-<network_id>) empty. External (baremetal) ports with OVN LSP type=external on that network reference this HCG; with it empty, no chassis owns them and ARP/routing breaks for all baremetal nodes on the network.

Root cause: for a vxlan-type external gateway neutron pins the Logical_Router to one chassis via options:chassis and creates a per-router HCG (neutron-<router_id>) carrying that chassis, but never sets ha_chassis_group on the gateway LRP. neutron's link_network_ha_chassis_group (called on internal LRP creation) bails at its if not gw_lrps[0].ha_chassis_group: return guard, so it never copies the chassis into neutron-<network_id>.

Fix: subscribe to ROUTER_INTERFACE/AFTER_CREATE at PRIORITY_DEFAULT+1000 (after OVN's handler has created the LRP) with a new handler link_vxlan_network_ha_chassis_group.

The handler:

1. Detects a vxlan-type gateway by checking that the per-router HCG (neutron-<router_id>) exists and has non-empty ha_chassis; VLAN/FLAT type gateways have no router HCG and continue to be handled by neutron unchanged, even though we are not using them.
2. Calls ovn_utils.sync_ha_chassis_group_network_unified to populate the per-network HCG (neutron-<network_id>) from the router HCG chassis — this is what fixes external/baremetal ports that already reference it.
3. Anchors the internal router-interface LRP (lrp-<port_id>) to the same unified network HCG so the router port is also correctly owned.

Both operations run in a single OVN NB transaction. The create_port_postcommit hook is unaffected — the internal LRP does not exist at that point (it is created later, on the same AFTER_CREATE event OVN processes first).

[mfencik]

LGTM