fix(alertmanager): quote null receiver and wire telegram template

Unquoted null parses as YAML null and breaks routing. Add shared telegram message template.

prometheus-stack/k8s/alertmanager-config.example.yaml

@@ -1,3 +1,6 @@
+templates:
+  - "/etc/alertmanager/config/telegram.tmpl"
+
 route:
   receiver: telegram
   group_by:
@@ -7,10 +10,10 @@ route:
   group_interval: 5m
   repeat_interval: 12h
   routes:
-    - receiver: null
+    - receiver: "null"
       matchers:
         - alertname="InfoInhibitor"
-    - receiver: null
+    - receiver: "null"
       matchers:
         - alertname="Watchdog"
 
@@ -35,4 +38,5 @@ receivers:
         chat_id: REPLACE_WITH_TELEGRAM_CHAT_ID
         parse_mode: HTML
         send_resolved: true
-  - name: null
+        message: '{{ template "telegram.forust.message" . }}'
+  - name: "null"