Add backend validation check for discrete question bin limit

Enforce the 200-outcome limit server-side in QuestionWriteSerializer.validate()
to prevent bypassing the frontend validation via direct API calls.

Co-authored-by: Sylvain <SylvainChevalier@users.noreply.github.com>

Author: github-actions[bot]

questions/serializers/common.py

@@ -217,6 +217,17 @@ def validate(self, data: dict):
                 errors.append("Range Max is required for continuous questions")
             if data.get("range_min") is None:
                 errors.append("Range Min is required for continuous questions")
+        if question_type == Question.QuestionType.DISCRETE:
+            inbound_outcome_count = data.get("inbound_outcome_count")
+            if (
+                inbound_outcome_count is not None
+                and inbound_outcome_count > DEFAULT_INBOUND_OUTCOME_COUNT
+            ):
+                errors.append(
+                    f"Discrete questions cannot have more than "
+                    f"{DEFAULT_INBOUND_OUTCOME_COUNT} outcomes. "
+                    f"Consider using a continuous question type instead."
+                )
 
         if errors:
             raise serializers.ValidationError(errors)