fix tests

Author: mmtftr

backend/src/test/java/com/group1/programminglanguagesforum/Services/TagServiceTests.java

@@ -142,6 +142,7 @@ void testGetTagDetails_Success() {
 
         when(tagRepository.findById(tagId)).thenReturn(Optional.of(mockTag));
         when(questionRepository.findQuestionsByTagId(tagId)).thenReturn(mockQuestions);
+        when(questionRepository.findQuestionsByDifficultyAndTagId(tagId)).thenReturn(mockQuestions);
 
         // Mocking modelMapper behavior
         when(modelMapper.map(any(Question.class), eq(GetQuestionWithTagDto.class)))