Adds a no-code routing layer to the OpenAI-compatible API server so one
Hermes deployment can map different API clients to different
model/provider backends. Clients pick a backend by sending a configured
alias as the OpenAI 'model' field; unmatched values fall back to the
global model. Configured aliases are listed by GET /v1/models.
Precedence (highest first): session /model override > model_routes
route > global config. Route provider credentials resolve through
_resolve_runtime_agent_kwargs_for_provider (same seam as
channel_overrides); per-route api_key/base_url are upstream provider
credential overrides — never caller auth, never logged.
Salvaged and rebased from PR #3176 by @Mibayy onto current main.
4a09b692ec