Function prefix_caching_active 

pub fn prefix_caching_active(
    enable_flag: bool,
    kv_only_prefix_cache_is_safe: bool,
) -> bool

2026-09-25: Whether the prefix cache the server will install is a real cache: the flag and ModelConfig::kv_only_prefix_cache_is_safe, the predicate build_prefix_cache uses. Preflight calls this before the cache exists; TransformerModel::new asks the constructed cache (PrefixCache::is_active).