add basic versions of retrieving answers by question or completion

SurveyBackend/SurveyBackend.Services/Services/AnswerVariantsService.cs

@@ -22,6 +22,14 @@ public class AnswerVariantsService : IAnswerVariantsService
     public async Task UpdateAnswerVariantAsync(AnswerVariant answerVariant)
     {
         // TODO: опять проверка существования, но еще и варианта
+        var answerVariantBase = await _answerVariantsRepository.GetByIdAsNoTrackingAsync(answerVariant.Id);
+        if (answerVariantBase is null)
+        {
+            throw new Exception("Answer Variant not found");
+        }
+
+        answerVariant.QuestionId = answerVariantBase.QuestionId;
+
         await _answerVariantsRepository.UpdateAsync(answerVariant);
     }