fix: treat GOOGLE_GENAI_USE_VERTEXAI=false as disabled

pkg/model/provider/gemini/client.go currently enables the Vertex AI path when GOOGLE_GENAI_USE_VERTEXAI is merely present, regardless of its value. Therefore GOOGLE_GENAI_USE_VERTEXAI=false can still route a Google-typed custom provider with an explicit base_url and token_key to the Vertex/ADC path, where token_key is not used.

This also disagrees with pkg/config/gather.go, which treats the setting as enabled only when non-empty.

Expected behavior: interpret GOOGLE_GENAI_USE_VERTEXAI as a boolean (or at minimum do not enable Vertex for false), align client routing with configuration validation, and add regression coverage for a custom Google provider using base_url + token_key when the variable is false.