An explicit empty string is a caller mistake, so it must reach the configuration
check and be reported as missing, rather than being silently replaced by
CREWAI_API_URL/CREWAI_BEARER_TOKEN. Adds a regression test and docstrings for the
test functions flagged by the docstring coverage check.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>