Fix prepared returns cache metadata collisions

[mameikagou]

Summary

• include pandas container type and preserved metadata in _prepare_returns() cache keys
• keep single-column DataFrame HTML reports working after cache isolation
• add regression coverage for both cache call orders and label preservation

Testing

• uv run --with pytest --with tzdata python -m pytest -s tests/ -q (129 passed)
• git diff --check

Closes #543