VRF router interface attach does not sync bound baremetal port physnets

Start with neutron_understack/tests/scenarios/test_vrf_router_interface.py::TestVrfRouterInterface::test_vrf_router_interface_syncs_bound_port_physnets and inspect the VRF router-interface path in neutron_understack_mech/routers. Run scenario VRF-RTR-01 to confirm the missing sync calls for physnet1 and physnet2. Done means the strict xfail can be removed and the test passes with both bound-port physnets synchronized.

Summary

Attaching a VRF (flavored) router interface to a network does not reconcile
the switches carrying that network's already-bound baremetal ports. undersync
is never asked to sync those ports' physical_networks, so the fabric is left
unaware that the network now has a router.

Component: python/neutron-understack — neutron_understack_mech / routers
Severity: Medium — baremetal ports on a network keep stale switch config
after a router is attached, until some other event happens to re-sync them.

Details

When a network has baremetal ports bound to one or more physnets (each with a
dynamic VLAN segment), and a VRF router is then attached to that network on the
internal side, no undersync.sync(<physnet>) call is made for those physnets.

For a flavored (VRF) router, routers.create_port_postcommit returns early
(it skips the OVN/uplink path for flavored routers), and nothing on the
router-interface path syncs the physnets of the ports already on the network.
The router-interface port itself is not a baremetal port, so
update_port_postcommit does not sync either.

Impact

Baremetal ports already present on a network do not get their switches
reconciled when a VRF router is attached. The switch configuration for those
physnets can be stale relative to the new L3 topology until an unrelated event
triggers a resync.

Steps to reproduce

Covered by
neutron_understack/tests/scenarios/test_vrf_router_interface.py::TestVrfRouterInterface::test_vrf_router_interface_syncs_bound_port_physnets
(scenario VRF-RTR-01), currently @pytest.mark.xfail(strict=True):

1. Create a VXLAN network + subnet.
2. Bind two baremetal ports on the network to different physnets
   (physnet1, physnet2).
3. Create a VRF router and attach the subnet on the internal side.
4. Expected: undersync.sync is called for physnet1 and physnet2.
   Actual: undersync.sync is not called for either.

Fix direction

On VRF router-interface attach, sync each physnet carrying a bound baremetal
port on the network so the switches are reconciled for the new router. When
fixed, remove the xfail on VRF-RTR-01 (strict xfail will otherwise start
failing to remind you).