Fix top level async breaks with routes that don't stream

[blittle]

Description

Fix a critical issue where a top level asynchronous request within App.server.jsx would break if subsequent routes attempt to disable streaming. For example:

// App.server.jsx
export default renderHydrogen() {
  useSession(); 
  ...
}

// NotFound.server.jsx
export default function NotFound({response}) {
  resonse.doNotStream();
  ...
}

Additional context

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