chore: improve deer-flow maintenance path

[lphuc2250gma]

Summary:

• Add or tighten focused edge-case tests or type assertions in backend/tests/conftest.py, backend/tests/test_auth.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.

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