fix(model-cache): dedupe concurrent fetches and throttle empty-response telemetry (#1072)

* fix(model-cache): dedupe concurrent fetches and throttle empty-response telemetry

* fix(model-cache): preserve per-caller failure contract on shared in-flight fetch

* refactor(model-cache): inline redundant refreshPromise IIFE in refreshModels

* fix(model-cache): drop redundant telemetry gate, restore ordering comment, cover in-flight cleanup

* test(model-cache): fix misleading auth-isolation comment, add same-token case

Author: edelauna