test: restore retry count assertion for RetryableQuotaError fallback

Address Gemini Code Assist review feedback: re-add mockFn call count
assertion to verify the retry logic attempts the expected number of
retries before triggering the fallback.

packages/core/src/utils/retry.test.ts

@@ -742,6 +742,8 @@ describe('retryWithBackoff', () => {
       AuthType.LOGIN_WITH_GOOGLE,
       expect.any(RetryableQuotaError),
     );
+    // 3 attempts (initial + 2 retries) fail, then fallback triggers, then 1 success
+    expect(mockFn).toHaveBeenCalledTimes(4);
   });
 
   it('should trigger fallback for OAuth personal users on ModelNotFoundError', async () => {