fix: resolve CI test failures - fix mock fixture name and complete assertions

tests/test_conversion.py

@@ -1,9 +1,8 @@
 """Tests for conversion service module."""
 
-import pytest
-from unittest.mock import MagicMock, patch
+from unittest.mock import MagicMock
 
-from codexchange.models import Language, ConversionRequest, ConversionResult
+from codexchange.models import Language
 from codexchange.services.conversion_service import ConversionService