NES: cold-start flicker before supportsUnifiedCompletions resolves on fetched /models path

Follow-up to #332802. On the fetched `/models` deployment path, `supportsUnifiedCompletions` resolves asynchronously, so there is a brief cold-start window (typically the `/models` round-trip, ~100ms) where the separate `github.copilot` completions provider is not yet excluded and ghost text can appear before `onModelListUpdated` fires and re-registers with the excludes.

Fully closing the window would require persisting the last-resolved `/models` config and hydrating it synchronously at startup (`ProxyModelsService` currently fetches fresh over the network with no disk cache), so warm starts have the correct excludes immediately. This does not affect local/ExP model configuration, which already resolves synchronously.