Fix CLI partial startup reporting when docling fails

[Bortlesboat]

Summary

• stop the CLI from printing full startup success when container startup or docling startup fails
• surface the docling failure reason when containers come up but docling does not
• add a focused CLI regression test for the partial-start and full-success cases

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 .venv/bin/python -m pytest tests/unit/test_cli.py --noconftest -q -p pytest_asyncio

Closes #1102