Export base_cli.testing or align testing docs

[codeforester]

Finding

docs/base-cli.md documents base_cli.testing.invoke(...), but lib/python/base_cli/__init__.py does not import or export the testing module. A caller following the docs with import base_cli gets AttributeError: module 'base_cli' has no attribute 'testing'.

Acceptance Criteria

• The documented testing helper access pattern works, or the docs require explicit from base_cli.testing import invoke.
• Add focused coverage so the package-level testing API/docs contract does not drift again.
• Existing base_cli.testing direct imports continue to work.

Source

Verified from the ninth-pass Claude review against current main.