feat(models): add push notification request models

- Add FirebaseRequestBody and OneSignalRequestBody models
- Include necessary JSON serialization and equity comparison
- Export new models from push_notification.dart

Author: fulleni

lib/src/models/models.dart

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+export 'push_notification/push_notification.dart';
+export 'request_id.dart';

lib/src/models/push_notification/firebase_request_body.dart

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+import 'package:equatable/equatable.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+part 'firebase_request_body.g.dart';
+
+/// {@template firebase_request_body}
+/// Represents the top-level structure for a Firebase Cloud Messaging
+/// v1 API request.
+/// {@endtemplate}
+@JsonSerializable(explicitToJson: true, includeIfNull: true, checked: true)
+class FirebaseRequestBody extends Equatable {
+  /// {@macro firebase_request_body}
+  const FirebaseRequestBody({required this.message});
+
+  /// The message payload.
+  final FirebaseMessage message;
+
+  /// Converts this [FirebaseRequestBody] instance to a JSON map.
+  Map<String, dynamic> toJson() => _$FirebaseRequestBodyToJson(this);
+
+  @override
+  List<Object> get props => [message];
+}
+
+/// {@template firebase_message}
+/// Represents the message object within a Firebase request.
+/// {@endtemplate}
+@JsonSerializable(explicitToJson: true, includeIfNull: true, checked: true)
+class FirebaseMessage extends Equatable {
+  /// {@macro firebase_message}
+  const FirebaseMessage({
+    required this.token,
+    required this.notification,
+    required this.data,
+  });
+
+  /// The registration token of the device to send the message to.
+  final String token;
+
+  /// The notification content.
+  final FirebaseNotification notification;
+
+  /// The custom data payload.
+  final Map<String, dynamic> data;
+
+  /// Converts this [FirebaseMessage] instance to a JSON map.
+  Map<String, dynamic> toJson() => _$FirebaseMessageToJson(this);
+
+  @override
+  List<Object> get props => [token, notification, data];
+}
+
+/// {@template firebase_notification}
+/// Represents the notification content within a Firebase message.
+/// {@endtemplate}
+@JsonSerializable(explicitToJson: true, includeIfNull: true, checked: true)
+class FirebaseNotification extends Equatable {
+  /// {@macro firebase_notification}
+  const FirebaseNotification({
+    required this.title,
+    required this.body,
+    this.image,
+  });
+
+  /// The notification's title.
+  final String title;
+
+  /// The notification's body text.
+  final String body;
+
+  /// The URL of an image to be displayed in the notification.
+  final String? image;
+
+  /// Converts this [FirebaseNotification] instance to a JSON map.
+  Map<String, dynamic> toJson() => _$FirebaseNotificationToJson(this);
+
+  @override
+  List<Object?> get props => [title, body, image];
+}

lib/src/models/push_notification/onesignal_request_body.dart

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+import 'package:equatable/equatable.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+part 'onesignal_request_body.g.dart';
+
+/// {@template onesignal_request_body}
+/// Represents the request body for the OneSignal /notifications endpoint.
+/// {@endtemplate}
+@JsonSerializable(
+  explicitToJson: true,
+  includeIfNull: false, // Do not include null fields in the JSON
+  checked: true,
+  fieldRename: FieldRename.snake,
+)
+class OneSignalRequestBody extends Equatable {
+  /// {@macro onesignal_request_body}
+  const OneSignalRequestBody({
+    required this.appId,
+    required this.includePlayerIds,
+    required this.headings,
+    required this.contents,
+    required this.data,
+    this.bigPicture,
+  });
+
+  /// The OneSignal App ID.
+  final String appId;
+
+  /// A list of OneSignal Player IDs to send the notification to.
+  final List<String> includePlayerIds;
+
+  /// The notification's title.
+  final Map<String, String> headings;
+
+  /// The notification's content.
+  final Map<String, String> contents;
+
+  /// The custom data payload.
+  final Map<String, dynamic> data;
+
+  /// The URL of a large image to display in the notification.
+  final String? bigPicture;
+
+  /// Converts this [OneSignalRequestBody] instance to a JSON map.
+  Map<String, dynamic> toJson() => _$OneSignalRequestBodyToJson(this);
+
+  @override
+  List<Object?> get props => [
+    appId,
+    includePlayerIds,
+    headings,
+    contents,
+    data,
+    bigPicture,
+  ];
+}

lib/src/models/push_notification/push_notification.dart

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+export 'firebase_request_body.dart';
+export 'onesignal_request_body.dart';