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/_private_artifact.py

@@ -12,9 +12,12 @@ from ._managed_files import DirectorySnapshot, FileSnapshot
 from ._pinned_directory import PinnedDirectory
 
 
+_RMDIR_SUPPORTS_DIR_FD = os.rmdir in os.supports_dir_fd
+
+
 def _remove_posix_private_directory(parent_descriptor: int, name: str) -> None:
     """Remove one empty child through the platform's native *at semantics."""
-    if os.rmdir not in os.supports_dir_fd:
+    if not _RMDIR_SUPPORTS_DIR_FD:
         raise OSError("Host does not support identity-bound directory removal")
     os.rmdir(name, dir_fd=parent_descriptor)
 

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 (