When provider: custom is set in config.yaml with base_url and api_key,
those values are now used instead of falling back to OPENAI_BASE_URL and
OPENAI_API_KEY env vars. Also reads the 'api' field as an alternative to
'api_key' for config compatibility.
Cherry-picked from PR #1762 by crazywriter1.
Co-authored-by: crazywriter1 <53251494+crazywriter1@users.noreply.github.com>