Resolve a documented cache policy for one effective model invocation.
Policies apply only when the endpoint is the provider's official API or a
user-declared trusted endpoint (see load_trusted_cache_endpoints).
A route that crosses wire formats (e.g. an OpenAI-format request routed to
an Anthropic model, spelled openai:anthropic/claude-...) resolves nothing,
because the translation such a hop requires rewrites or drops the caching
fields the policy assumes. See _effective_model_name. Cross-format routing
that is not spelled in the model name cannot be detected -- declaring an
endpoint trusted asserts that it does not do this.
Resolved endpoint, or None for the provider default.
Hostnames the user has declared trusted, as returned
by load_trusted_cache_endpoints. Full http(s) URLs are accepted
too and reduced to their host; entries that are not a usable
hostname are dropped, exactly as the loader drops them. Matching is
per exact host -- a trusted example.com does not cover
gw.example.com.
