Introduce unit testing on calculation objects

Tests/Builders/StudentEventRankingBuilder.cs

@@ -0,0 +1,65 @@
+using Core.Entities;
+
+namespace Tests.Builders;
+
+/// <summary>
+/// Fluent builder for creating StudentEventRanking test entities.
+/// </summary>
+public class StudentEventRankingBuilder
+{
+    private Student? _student = null;
+    private EventDefinition? _eventDefinition = null;
+    private int _rank = 1;
+
+    public StudentEventRankingBuilder ForStudent(Student student)
+    {
+        _student = student;
+        return this;
+    }
+
+    public StudentEventRankingBuilder ForEvent(EventDefinition eventDef)
+    {
+        _eventDefinition = eventDef;
+        return this;
+    }
+
+    public StudentEventRankingBuilder WithRank(int rank)
+    {
+        if (rank < 1 || rank > StudentEventRanking.MaxRank)
+            throw new ArgumentOutOfRangeException(nameof(rank),
+                $"Rank must be between 1 and {StudentEventRanking.MaxRank}");
+
+        _rank = rank;
+        return this;
+    }
+
+    public StudentEventRanking Build()
+    {
+        if (_student == null)
+            throw new InvalidOperationException("StudentEventRanking must have a student. Call ForStudent() before Build().");
+
+        if (_eventDefinition == null)
+            throw new InvalidOperationException("StudentEventRanking must have an event. Call ForEvent() before Build().");
+
+        var ranking = new StudentEventRanking
+        {
+            Student = _student,
+            EventDefinition = _eventDefinition,
+            Rank = _rank
+        };
+
+        // Set up bidirectional relationship: add to student's EventRankings if not already there
+        if (!_student.EventRankings.Any(er => er.EventDefinition == _eventDefinition))
+        {
+            _student.EventRankings.Add(ranking);
+        }
+
+        return ranking;
+    }
+
+    // Static factory methods
+    public static StudentEventRankingBuilder Default() => new StudentEventRankingBuilder();
+
+    public static StudentEventRankingBuilder Create(Student student, EventDefinition eventDef) =>
+        new StudentEventRankingBuilder().ForStudent(student).ForEvent(eventDef);
+}