Wait for HA peer DNS before starting kea-dhcp4

## Why
kea-dhcp4 crash-loops on a cold container start: the HA hook resolves the StatefulSet peer URL hostnames once at config load, but the peer DNS records are not resolvable in the first moment of a fresh container, and kea exits hard instead of retrying (HA_CONFIGURATION_FAILED / "Failed to convert string to address"). Once DNS is warm the exact config validates, so the failure is purely a startup race.

## How
- gate the dhcp4 entrypoint on `kea-dhcp4 -t` and retry until the config validates before exec'ing the server

internal/kea/agent.go

@@ -37,6 +37,15 @@ ORD="${HOSTNAME##*-}"
 mkdir -p %[1]s
 chmod 0750 %[1]s
 sed "s/%[2]s/server${ORD}/g" %[3]s/kea-dhcp4.conf > %[4]s
+# The HA hook resolves peer URL hostnames once at load; on a cold container
+# start the StatefulSet peer DNS records may not resolve yet, and kea exits
+# hard instead of retrying. Wait for the config to validate before starting.
+i=0
+until %[5]s -t %[4]s >/dev/null 2>&1; do
+  i=$((i+1))
+  if [ "$i" -ge 60 ]; then break; fi
+  sleep 2
+done
 exec %[5]s -c %[4]s
 `, RunDir, ThisServerPlaceholder, ConfigDir, DHCP4ConfPath, DHCP4Bin)
 }