chore: improve bup maintenance path - #76

Summary:

• Add or tighten focused edge-case tests or type assertions in test/int/test_io.py, test/ext/test_ftp.py related to Python typing, tests, CLI ergonomics, observability; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.