Function build_openai_prompt_cache_key 

pub fn build_openai_prompt_cache_key(
    prompt_cache_enabled: bool,
    prompt_cache_key_mode: &OpenAIPromptCacheKeyMode,
    lineage_id: Option<&str>,
) -> Option<String>

Build a stable OpenAI prompt_cache_key for requests that should share provider-side cache routing.