Function resolve_provider_cache_root 

pub fn resolve_provider_cache_root(provider: &str) -> PathBuf

Resolve the root cache directory for a provider (no model sub-directory).

Used when setting a process-global cache env var (e.g. HF_HOME for mistralrs) before the first model load.

Priority:

1. UNI_CACHE_DIR env var – resolves to $UNI_CACHE_DIR/<provider>
2. .uni_cache/<provider> – default