added existance check (not sure if it's working)

SurveyBackend/SurveyBackend.Services/Services/QuestionService.cs

@@ -22,7 +22,12 @@ public class QuestionService : IQuestionService
 
     public async Task AddQuestionAsync(QuestionBase question)
     {
-        // TODO: проверить существование опроса
+        var survey = await _surveyRepository.GetByIdAsync(question.SurveyId);
+        if (survey is null)
+        {
+            throw new NotFoundException("Survey not found");
+        }
+
         await _questionRepository.AddAsync(question);
     }