Function apply_default_headers

modules/llrt_fetch/src/fetch.rs:420–436

(
    req: &mut hyper::http::request::Builder,
    detected_headers: &HashSet<&str>,
)

            headers,
            body,
            &response_status,
            initial_uri,
        )?;

        let res = dispatch(ctx, client, req, abort_receiver).await?;

        let status = res.status();

        // Per WHATWG Fetch §4.5 step 15.4: 421 on a non-GET/HEAD request retries once on a fresh connection.
        if status.as_u16() == 421 && !matches!(method, Method::GET | Method::HEAD) {
            drop(res);
            // Force a new connection: open a new hyper client whose pool can't share TCP conns with the global one.
            let new_client = llrt_http::build_client(None)
                .or_throw_type(ctx, "Fetch failed: could not build retry client")?;
            let (req, _) = build_request(
                ctx,
                &method,
                uri,
                headers,
                body,