return mascotMood on answer submission

The math-app frontend prototype drives a RobotMascot with a fixed set of
moods; this gives that animation an explicit backend signal. Complements
the existing Lesson.mascot_animation / LessonIntro.mascotAnimation, which
are unchanged.

serializers.py
  - AnswerResponseSerializer: new mascotMood field.
views.py
  - QuestionAnswerView: mascotMood = 'happy' for a correct answer else
    'encourage', on both the fresh response and the idempotent-replay
    response.
tests.py
  - QuestionAnswerMascotMoodTests: correct -> happy, wrong -> encourage,
    and the mood is preserved across an idempotent replay.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/serializers.py

@@ -79,6 +79,9 @@ class AnswerResponseSerializer(serializers.Serializer):
     correctOptionId = serializers.CharField()
     xpEarned = serializers.IntegerField()
     heartsRemaining = serializers.IntegerField()
+    # حالتِ انیمیشنِ ماسکوت («رُبو») برای فرانت: جوابِ درست → happy، غلط →
+    # encourage. هم‌خوان با RobotMood در MathApp_frontend.
+    mascotMood = serializers.CharField()
 
 
 # ---- Lesson completion ----