Tests and updates the readiness of a saved runtime LLM (admin).

Tests the stored UI/API runtime configuration identified by registry ID. A successful test makes the model serving-eligible. A failed test preserves the configuration and records probe_failed, the error, and the test time; a probe_failed configuration is re-probed automatically at startup, so it can become serving-eligible later without another manual test. Returns 200 with ok=false for a model test failure. A core transport or invalid-response error leaves the prior readiness unchanged. No request body is accepted; the saved configuration is used. Admin only.