Don't show average grade if grading system is not numerical. (#1442)

Don't show the calculated average grade if the grading system is not
numerical. To be able to do this I added `GradeResult.gradingSystem` and
`GradeValue.gradingSystem`.

Before:

![image](https://github.com/SharezoneApp/sharezone-app/assets/29028262/d0eae3a3-f5db-4c0c-ba6d-0d0a2032e36d)

After:

![image](https://github.com/SharezoneApp/sharezone-app/assets/29028262/8ab3adf2-5d30-45e7-a54d-3711070c08a4)
 
Requires #1441

---------

Co-authored-by: Jonas <--help>

app/lib/grades/grades_service/grades_service.dart

@@ -385,6 +385,7 @@ class GradeResult {
   final GradeValue value;
   final bool isTakenIntoAccount;
   final Date date;
+  GradingSystem get gradingSystem => value.gradingSystem;
 
   GradeResult({
     required this.id,
@@ -447,6 +448,8 @@ class GradeValue extends Equatable {
   double get asDouble => asNum.toDouble();
   final num asNum;
 
+  final GradingSystem gradingSystem;
+
   /// Only available if there is a special displayable grade for the calculated
   /// grade. For example, if the calculated grade is 2.25, the displayable grade
   /// could be '2+' for the (1-6 with +-) grading system.
@@ -457,12 +460,14 @@ class GradeValue extends Equatable {
   final String? suffix;
 
   @override
-  List<Object?> get props => [asNum, displayableGrade, suffix];
+  List<Object?> get props => [asNum, gradingSystem, displayableGrade, suffix];
 
-  const GradeValue(
-      {required this.asNum,
-      required this.displayableGrade,
-      required this.suffix});
+  const GradeValue({
+    required this.asNum,
+    required this.gradingSystem,
+    required this.displayableGrade,
+    required this.suffix,
+  });
 }
 
 class Grade {

app/lib/grades/grades_service/src/grading_systems.dart

@@ -81,6 +81,7 @@ class _GradingSystem {
   GradeValue toGradeResult(num grade) {
     return GradeValue(
       asNum: grade,
+      gradingSystem: toGradingSystems(),
       displayableGrade: spec.getSpecialDisplayableGradeOrNull?.call(grade),
       suffix: spec == zeroToHundredPercentWithDecimalsSpec ? '%' : null,
     );

app/lib/grades/pages/grades_page/grades_page_controller.dart

@@ -97,6 +97,9 @@ class GradesPageController extends ChangeNotifier {
 // This is not covered by tests yet.
 String displayGrade(GradeValue? grade) {
   if (grade == null) return '—';
+  if (!grade.gradingSystem.isNumericalAndContinous) {
+    return grade.displayableGrade ?? '—';
+  }
 
   String withSuffix(String gs) => '$gs${grade.suffix ?? ''}';
 

app/test/grades/grades_test.dart

@@ -704,6 +704,45 @@ void main() {
               .date,
           Date('2024-03-26'));
     });
+    test('A grade and GradeValue has a gradingSystem', () {
+      final controller = GradesTestController();
+
+      final term = termWith(
+        subjects: [
+          subjectWith(
+            id: const SubjectId('Philosophie'),
+            grades: [
+              gradeWith(
+                id: GradeId('grade1'),
+                gradingSystem: GradingSystem.oneToFiveWithDecimals,
+                value: 2.5,
+              ),
+              gradeWith(
+                id: GradeId('grade2'),
+                gradingSystem: GradingSystem.sixToOneWithDecimals,
+                value: 4.2,
+              ),
+            ],
+          ),
+        ],
+      );
+      controller.createTerm(term);
+
+      final grade1 = controller
+          .term(term.id)
+          .subject(const SubjectId('Philosophie'))
+          .grade(GradeId('grade1'));
+      // On both since the calculated averageGrade is only a [GradeValue], not a [GradeResult].
+      expect(grade1.gradingSystem, GradingSystem.oneToFiveWithDecimals);
+      expect(grade1.value.gradingSystem, GradingSystem.oneToFiveWithDecimals);
+
+      final grade2 = controller
+          .term(term.id)
+          .subject(const SubjectId('Philosophie'))
+          .grade(GradeId('grade2'));
+      expect(grade2.gradingSystem, GradingSystem.sixToOneWithDecimals);
+      expect(grade2.value.gradingSystem, GradingSystem.sixToOneWithDecimals);
+    });
     test('A subject has a Design', () {
       final controller = GradesTestController();