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/config/test_config_loader.py

@@ -12,7 +12,6 @@ Tests cover:
 from __future__ import annotations
 
 import pytest
-import os
 import json
 
 from langbot.pkg.config.impls.yaml import YAMLConfigFile

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
 
 

tests/unit_tests/pipeline/test_preproc.py

@@ -20,7 +20,6 @@ from tests.factories import (
     text_query,
     empty_query,
     image_query,
-    unsupported_query,
     group_text_query,
 )
 
@@ -198,7 +197,6 @@ class TestPreProcessorImageSegment:
         # Image query with base64
         query = image_query(text="look at this", url=None)
         # Set base64 on the image component
-        from tests.factories.message import image_chain
         import langbot_plugin.api.entities.builtin.platform.message as platform_message
         chain = platform_message.MessageChain([
             platform_message.Plain(text="look at this"),