denyFromManagement returned the generic "LLM policy limit exceeded" for
every deny code. Map llm_policy.model_blocked / model_unknown to the same
model-specific messages the local guardrail uses, keeping other codes
generic so internal quota detail never leaks.
Document the fail-open trade-off: the proxy backstop carries the union of
restricting policies' models, so per-group narrowing is lost during a
CheckLLMPolicyLimits outage.
7cd1cda70e