load_model answers not_found while a kserve backend is still installing #312

Problem

While a newly registered kserve backend's llm-d controller is still installing, list_presets returns an empty list and load_model answers that the model is "neither cached, served nor a preset". The model is a valid preset; the backend is just not ready yet. The not_found answer sends the caller looking for a wrong model name instead of waiting for the backend.

Proposed solution

When the backend's serving stack (the llm-d controller and its presets) is not ready, list_presets and load_model answer with a backend-not-ready reason that names the backend and what it waits on, instead of an empty list or not_found.

Acceptance criteria

• During the controller's installation load_model of a preset model answers a backend-not-ready reason, not not_found.
• list_presets says the backend is not ready instead of returning an empty list.
• Once the backend is ready the same calls behave as today.

Written by an agent.