test(auto-update): isolate default manager detection

Keep the fallback-to-npm assertion independent from a runner's npm_config_user_agent. This matches the test's custom-prefix scenario while preserving runtime package-manager detection.

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Author: sunnylqm

tests/auto-update.test.ts

@@ -67,6 +67,7 @@ describe('auto update package-manager detection', () => {
     expect(
       detectGlobalPackageManager({
         packageRoot: '/custom/prefix/lib/node_modules/react-native-update-cli',
+        env: {},
       }),
     ).toBe('npm');
   });