fix(core): ensure stable fallback for restricted preview models (#26999)

packages/core/src/fallback/handler.ts

@@ -20,6 +20,8 @@ import {
   applyAvailabilityTransition,
 } from '../availability/policyHelpers.js';
 
+import { isPreviewModel } from '../config/models.js';
+
 export const UPGRADE_URL_PAGE = 'https://goo.gle/set-up-gemini-code-assist';
 
 export async function handleFallback(
@@ -28,13 +30,19 @@ export async function handleFallback(
   authType?: string,
   error?: unknown,
 ): Promise<string | boolean | null> {
+  const failureKind = classifyFailureKind(error);
+
+  // If a preview model is not found, record that the user lacks preview access.
+  if (failureKind === 'not_found' && isPreviewModel(failedModel, config)) {
+    config.setHasAccessToPreviewModel?.(false);
+  }
+
   const chain = resolvePolicyChain(config);
   const { failedPolicy, candidates } = buildFallbackPolicyContext(
     chain,
     failedModel,
   );
 
-  const failureKind = classifyFailureKind(error);
   const availability = config.getModelAvailabilityService();
   const getAvailabilityContext = () => {
     if (!failedPolicy) return undefined;