feat: Implement trip invitation functionality and notification handling

- Added TripInvitationRepository for managing trip invitations.
- Created TripInvitation model with serialization methods.
- Implemented notification payload parser for handling FCM notifications.
- Enhanced NotificationService to manage trip invitations and related actions.
- Updated UserRepository to include user search functionality.
- Modified AuthRepository to store multiple FCM tokens.
- Added tests for trip invitation logic and notification payload parsing.
- Updated pubspec.yaml and pubspec.lock for dependency management.

test/models/trip_invitation_test.dart

@@ -0,0 +1,49 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:travel_mate/models/trip_invitation.dart';
+
+void main() {
+  group('TripInvitation', () {
+    test('toMap serializes required fields', () {
+      final invitation = TripInvitation(
+        id: 'id_1',
+        tripId: 'trip_1',
+        tripTitle: 'Weekend Rome',
+        inviterId: 'user_a',
+        inviterName: 'Alice',
+        inviteeId: 'user_b',
+        inviteeEmail: 'bob@example.com',
+        createdAt: DateTime(2026, 3, 3),
+      );
+
+      final map = invitation.toMap();
+
+      expect(map['tripId'], 'trip_1');
+      expect(map['status'], 'pending');
+      expect(map['respondedAt'], isNull);
+      expect(map['createdAt'], isA<Timestamp>());
+    });
+
+    test('copyWith updates selected values', () {
+      final invitation = TripInvitation(
+        id: 'id_1',
+        tripId: 'trip_1',
+        tripTitle: 'Trip',
+        inviterId: 'user_a',
+        inviterName: 'Alice',
+        inviteeId: 'user_b',
+        inviteeEmail: 'bob@example.com',
+        createdAt: DateTime(2026, 3, 3),
+      );
+
+      final updated = invitation.copyWith(
+        status: 'accepted',
+        respondedAt: DateTime(2026, 3, 4),
+      );
+
+      expect(updated.status, 'accepted');
+      expect(updated.respondedAt, DateTime(2026, 3, 4));
+      expect(updated.tripId, invitation.tripId);
+    });
+  });
+}