Improve code to cover full if/else branches

[huan]

apply changes based on this feedback

The new GOOGLE_GENAI_USE_VERTEXAI variable is set automatically when gh CLI is available, but it's not included in the manual setup instructions in the else block (lines 462-477). Users without the gh CLI won't know to set this variable manually. Consider adding documentation for this variable in the else block, similar to the other variables.