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

tests/test_pinned_directory.py

@@ -21,7 +21,7 @@ def test_darwin_private_directory_removal_uses_stdlib_dir_fd(
         calls.append((path, dir_fd))
 
     monkeypatch.setattr(_private_artifact.os, "rmdir", record_rmdir)
-    monkeypatch.setattr(_private_artifact.os, "supports_dir_fd", {record_rmdir})
+    monkeypatch.setattr(_private_artifact, "_RMDIR_SUPPORTS_DIR_FD", True)
 
     _private_artifact._remove_posix_private_directory(41, "private-container")