Add ReportRequestBody model with JSON serialization

This commit introduces a new Dart class, `ReportRequestBody`, which represents the request body for generating reports in the `BenchmarkService`. The class includes a `toJson` method for easy serialization to JSON format.

- `category`: Specifies the category of the report (e.g., "coding").
- `tests`: A list of tests to be included in the report.

frontend/lib/models/benchmark_service/report_request_body.dart

@@ -0,0 +1,13 @@
+class ReportRequestBody {
+  final String category;
+  final List<String> tests;
+
+  ReportRequestBody({required this.category, required this.tests});
+
+  Map<String, dynamic> toJson() {
+    return {
+      'category': category,
+      'tests': tests,
+    };
+  }
+}