fix(mailercloud): keep _domainkey and _dmarc as fixed literals in template

Address review: dkim host -> %dkim_selector%._domainkey%dkim_suffix% and
dmarc host -> _dmarc%dmarc_suffix%, so the well-known ._domainkey / _dmarc
labels are fixed literals; only the selector and the optional sending-
subdomain suffix remain variable.

Author: arunkvinam

mailercloud.com.email-auth.json

@@ -9,7 +9,7 @@
   "syncPubKeyDomain": "dns-connect.mailercloud.com",
   "syncRedirectDomain": "app.mailercloud.com",
   "multiInstance": true,
-  "variableDescription": "cloudnskey: per-account tracking key. track_host/dkim_host/img_host/spf_host/dmarc_host: server-computed relative hosts (apex mc / k1._domainkey / img<key> / @ / _dmarc; subdomain adds the sub-label; occupied-subdomain tracking mc-<label>). img_value: img<key>.<imgdomain>. dmarc_value: server-computed DMARC TXT (merged with existing policy, or default policy with rua/ruf@trackdomain).",
+  "variableDescription": "cloudnskey: per-account tracking key. track_host/img_host/spf_host: server-computed relative hosts (apex mc / img<key> / @; subdomain adds the sub-label; occupied-subdomain tracking mc-<label>). dkim_selector: DKIM selector (e.g. k1). dkim_suffix: subdomain suffix after ._domainkey (empty at apex, .<sub> for a subdomain) — the ._domainkey label is fixed in the template. dmarc_suffix: subdomain suffix after _dmarc (empty at apex, .<sub>) — _dmarc is fixed in the template. img_value: img<key>.<imgdomain>. dmarc_value: server-computed DMARC TXT (existing policy preserved, with rua/ruf@trackdomain).",
   "records": [
     {
       "groupId": "tracking",
@@ -22,7 +22,7 @@
     {
       "groupId": "dkim",
       "type": "CNAME",
-      "host": "%dkim_host%",
+      "host": "%dkim_selector%._domainkey%dkim_suffix%",
       "pointsTo": "dkim.mlrcloud.com",
       "ttl": 3600,
       "essential": "Always"
@@ -46,7 +46,7 @@
     {
       "groupId": "dmarc",
       "type": "TXT",
-      "host": "%dmarc_host%",
+      "host": "_dmarc%dmarc_suffix%",
       "data": "%dmarc_value%",
       "ttl": 3600,
       "txtConflictMatchingMode": "All",