Model Inference Proxy GET

Proxy requests to model entity inference endpoints. All inference requests must resolve to a `VirtualModel`. The platform's provider reconciler auto-creates an implicit `autoprovisioned` VirtualModel for every served model entity (named after the entity, with `default_model_entity` set to the entity ref) so this is the typical case; operators can also create custom VirtualModels for routing, plugin chains, LoRA escape-hatches, etc. Requests for which no VirtualModel can be found return `404`.