test: stabilize app-dir hash navigation RSC assertion (#89867)
## Summary
- stabilize the hash navigation assertion in `navigation.test.ts` by
checking for query-param-specific RSC requests instead of any RSC
request
- switch request header reads from `req.allHeaders()` to synchronous
`req.headers()` in the request listener to avoid teardown timing errors
- keep the existing behavior check that query-param navigation triggers
an RSC request
