Return is_based_on_alertmanager for alert_receive_channel in private api (#1782)

# What this PR does

## Which issue(s) this PR fixes

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

engine/apps/api/serializers/alert_receive_channel.py

@@ -220,6 +220,7 @@ class AlertReceiveChannelTemplatesSerializer(EagerLoadingMixin, serializers.Mode
     ]
 
     payload_example = SerializerMethodField()
+    is_based_on_alertmanager = SerializerMethodField()
 
     class Meta:
         model = AlertReceiveChannel
@@ -227,6 +228,7 @@ class AlertReceiveChannelTemplatesSerializer(EagerLoadingMixin, serializers.Mode
             "id",
             "verbal_name",
             "payload_example",
+            "is_based_on_alertmanager",
         ]
         extra_kwargs = {"integration": {"required": True}}
 
@@ -246,6 +248,9 @@ class AlertReceiveChannelTemplatesSerializer(EagerLoadingMixin, serializers.Mode
             except AttributeError:
                 return None
 
+    def get_is_based_on_alertmanager(self, obj):
+        return obj.has_alertmanager_payload_structure
+
     # Override method to pass field_name directly in set_value to handle None values for WritableSerializerField
     def to_internal_value(self, data):
         """

engine/apps/api/tests/test_alert_receive_channel_template.py

@@ -365,6 +365,7 @@ def test_update_alert_receive_channel_templates(
     del existing_templates_data["id"]
     del existing_templates_data["verbal_name"]
     del existing_templates_data["payload_example"]
+    del existing_templates_data["is_based_on_alertmanager"]
 
     # update each template
     new_templates_data = {}