chore(python): audit tests

Split the three oversized test files into support modules and topic
files, evaluate the rmdir dir_fd capability once at import, and drop
unused imports.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

elydora/plugins/qwen_sources.py

@@ -10,7 +10,7 @@ import tempfile
 from typing import Dict, List, Optional, Set, Tuple
 
 from ._dotenv import parse_dotenv
-from ._managed_files import MAX_SOURCE_BYTES, FileSnapshot, read_physical_file
+from ._managed_files import MAX_SOURCE_BYTES, read_physical_file
 from ._runtime import same_path, unique_preconditions
 from ._transaction import FilePrecondition
 from .qwen_config import (