[ci_gen_kustomize_values] BGP: generate gateway IP for network bridges from CIDR

Use `ansible.utils.nthhost(1)` to derive the bridge gateway IP directly
from the network CIDR instead of requiring a hardcoded `gateway` field
in `values.yaml`. This ensures the `internalapi` and `storage` bridge
gateway addresses in BGP deployments are always consistent with the
configured subnet.

Required by the NNCP fix in the `architecture` repo (OSPRH-32725), where
the NNCP desired state must include the bridge gateway IP so NMState does
not remove it during periodic reconciliation cycles, which otherwise causes
`ovn-controller` to lose connectivity to the OVN SB DB pods.

Related-Issue: #OSPRH-32725

Assisted-By: Claude Sonnet 4.6
Signed-off-by: Eduardo Olivares <eolivare@redhat.com>

Author: eduolivares

roles/ci_gen_kustomize_values/templates/bgp-l3-xl/network-values/values.yaml.j2

@@ -108,6 +108,7 @@ data:
 {%       endfor %}
 {%     endif %}
 {%   endif %}
+    gateway: {{ network.network_v4 | ansible.utils.nthhost(1) }}
     prefix-length: {{ network.network_v4 | ansible.utils.ipaddr('prefix') }}
     mtu: {{ network.mtu | default(1500) }}
 {%   if network.vlan_id is defined  %}

roles/ci_gen_kustomize_values/templates/bgp_dt01/network-values/values.yaml.j2

@@ -114,6 +114,7 @@ data:
 {%      endfor %}
 {%    endif %}
 {%  endif %}
+    gateway: {{ network.network_v4 | ansible.utils.nthhost(1) }}
     prefix-length: {{ network.network_v4 | ansible.utils.ipaddr('prefix') }}
     mtu: {{ network.mtu | default(1500) }}
 {%  if network.vlan_id is defined  %}