Add testing for validation

Tests/Builders/StudentBuilder.cs

@@ -147,6 +147,7 @@ public class StudentBuilder
                 Rank = rank
             };
             student.EventRankings.Add(ranking);
+            student.RankedEvents.Add(eventDef); // Also add to RankedEvents for validation rules
         }
 
         return student;

Tests/Builders/TeamBuilder.cs

@@ -21,10 +21,12 @@ public class TeamBuilder
         return this;
     }
 
-    public TeamBuilder WithStudent(Student student)
+    public TeamBuilder WithStudent(Student student, bool isCaptain = false)
     {
         if (!_students.Contains(student))
             _students.Add(student);
+        if (isCaptain)
+            _captain = student;
         return this;
     }