Extend ReportRequestBody to Include "mock" Boolean Field

This commit adds a new boolean field, "mock", to the `ReportRequestBody` class. This additional field is in line with the new requirements to specify whether the report is a mock or not.

The `toJson()` method is also updated to include this new field during serialization.

frontend/lib/models/benchmark_service/report_request_body.dart

@@ -1,13 +1,16 @@
 class ReportRequestBody {
   final String category;
   final List<String> tests;
+  final bool mock;
 
-  ReportRequestBody({required this.category, required this.tests});
+  ReportRequestBody(
+      {required this.category, required this.tests, required this.mock});
 
   Map<String, dynamic> toJson() {
     return {
       'category': category,
       'tests': tests,
+      'mock': mock,
     };
   }
 }