CacheConfidence = Literal['expired', 'may_be_cold']How firmly a passed retention window implies the cached prefix is gone.
expired is used when the window is a documented maximum (Anthropic's TTL,
OpenAI's prompt_cache_retention ceilings): once it passes, the entry is gone.
may_be_cold is used when the window is a documented minimum (GPT-5.6+),
where the provider is only guaranteed to have kept the entry that long and may
well have kept it longer.