Introduce unit testing on calculation objects

Tests/Builders/TeamBuilder.cs

@@ -0,0 +1,100 @@
+using Core.Entities;
+
+namespace Tests.Builders;
+
+/// <summary>
+/// Fluent builder for creating Team test entities.
+/// </summary>
+public class TeamBuilder
+{
+    private static int _idCounter = 1;
+
+    private int _id = _idCounter++;
+    private EventDefinition? _event = null;
+    private List<Student> _students = new();
+    private Student? _captain = null;
+    private string? _identifier = null;
+
+    public TeamBuilder ForEvent(EventDefinition eventDef)
+    {
+        _event = eventDef;
+        return this;
+    }
+
+    public TeamBuilder WithStudent(Student student)
+    {
+        if (!_students.Contains(student))
+            _students.Add(student);
+        return this;
+    }
+
+    public TeamBuilder WithStudents(params Student[] students)
+    {
+        foreach (var student in students)
+        {
+            if (!_students.Contains(student))
+                _students.Add(student);
+        }
+        return this;
+    }
+
+    public TeamBuilder WithStudents(IEnumerable<Student> students)
+    {
+        foreach (var student in students)
+        {
+            if (!_students.Contains(student))
+                _students.Add(student);
+        }
+        return this;
+    }
+
+    public TeamBuilder WithCaptain(Student captain)
+    {
+        _captain = captain;
+        // Ensure captain is in the students list
+        if (!_students.Contains(captain))
+            _students.Add(captain);
+        return this;
+    }
+
+    public TeamBuilder WithIdentifier(string identifier)
+    {
+        _identifier = identifier;
+        return this;
+    }
+
+    public Team Build()
+    {
+        if (_event == null)
+            throw new InvalidOperationException("Team must have an event. Call ForEvent() before Build().");
+
+        var team = new Team
+        {
+            Id = _id,
+            Event = _event,
+            Students = _students.ToList(),
+            Captain = _captain,
+            Identifier = _identifier
+        };
+
+        // Set up bidirectional relationship: add team to each student's Teams collection
+        foreach (var student in _students)
+        {
+            if (!student.Teams.Contains(team))
+                student.Teams.Add(team);
+        }
+
+        return team;
+    }
+
+    // Static factory methods
+    public static TeamBuilder Default() => new TeamBuilder();
+
+    public static TeamBuilder Create(EventDefinition eventDef) =>
+        new TeamBuilder().ForEvent(eventDef);
+
+    /// <summary>
+    /// Reset the ID counter for test isolation. Call this in test SetUp.
+    /// </summary>
+    public static void ResetIdCounter() => _idCounter = 1;
+}