feat: functions.yaml has comment

Author: brianquinlan

lib/src/builder/manifest.dart

@@ -24,6 +24,9 @@ import 'package:yaml_edit/yaml_edit.dart';
 import '../common/cloud_run_id.dart';
 import 'spec.dart';
 
+const _manifestComment =
+    '# This file is generated by package:firebase_functions. Do not edit.';
+
 /// Generates the YAML manifest string from discovered params and endpoints.
 String generateManifestYaml(
   Map<String, ParamSpec> params,
@@ -32,7 +35,7 @@ String generateManifestYaml(
   final map = _buildManifestMap(params, endpoints);
   final editor = YamlEditor('');
   editor.update([], map);
-  return editor.toString();
+  return '$_manifestComment\n\n${editor.toString()}';
 }
 
 /// Builds the full manifest as a structured map.

test/snapshots/manifest_snapshot_test.dart

@@ -82,6 +82,13 @@ void main() {
       expect(dartManifest['specVersion'], equals('v1alpha1'));
     });
 
+    test('should have a comment at top', () {
+      final dartYaml = File(
+        'test/fixtures/dart_reference/functions.yaml',
+      ).readAsStringSync();
+      expect(dartYaml, startsWith('#'));
+    });
+
     // =========================================================================
     // Params Tests
     // =========================================================================