refactor(app): replace providers-snapshot invalidate with refetchIfStale

invalidate() flagged the query stale and kicked off a refetch unconditionally,
causing a loading flash every time the model selector opened. Expose
refetchIfStale() instead so callers only refetch when data is actually stale.

packages/app/src/hooks/use-agent-input-draft.ts

@@ -167,7 +167,7 @@ function buildDraftStatusControls(input: {
     features,
     onSetFeature,
     onDropdownClose,
-    onModelSelectorOpen: formState.invalidateProviderModels,
+    onModelSelectorOpen: formState.refetchProviderModelsIfStale,
   };
 }