feat(StudentCourse): Add != operator for use in testing

src/g2_2025_grade_calculator_pkg/src/database/StudentCourse.hpp

@@ -24,6 +24,10 @@ struct StudentCourse {
             || (student_name == other.student_name
                 && course_name < other.course_name);
     }
+
+    bool operator!=(const StudentCourse& other) const {
+        return !(*this == other);
+    }
 };
 
 typedef std::map<StudentCourse, std::vector<int>> StudentCourseResultMap;