Function get_google_llm_variant 

pub fn get_google_llm_variant() -> GoogleLlmVariant

Determine the Google LLM variant from the environment.

Reads the GOOGLE_GENAI_USE_VERTEXAI environment variable. Returns GoogleLlmVariant::VertexAi when the variable is set to a truthy value ("true", "1", case-insensitive), and GoogleLlmVariant::GeminiApi otherwise (including when the variable is unset).