style: fix unused imports after ruff auto-fix

Remove unused imports in test files:
- test_config_loader.py: remove unused os
- test_taskmgr.py: remove unused Mock
- test_preproc.py: remove unused unsupported_query, image_chain

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/unit_tests/core/test_taskmgr.py

@@ -16,7 +16,7 @@ from __future__ import annotations
 import pytest
 import asyncio
 import sys
-from unittest.mock import MagicMock, Mock
+from unittest.mock import MagicMock
 from importlib import import_module