shopify_draft_proxy/proxy/passthrough

Substrate for forwarding a DraftProxy request to upstream Shopify verbatim and returning the upstream response unchanged.

Lives here (separate from draft_proxy.gleam) so domain handlers can call passthrough_sync themselves when their per-operation logic decides upstream is the right answer — without importing draft_proxy and creating a cycle. The dispatcher in draft_proxy.gleam also calls this for its own substrate-level fallbacks (operations marked Passthrough in the registry; root fields with no local dispatcher implemented).

On the Erlang target, passthrough_sync always works: it uses the installed upstream_transport if any, otherwise the real upstream_client.send_sync HTTP shim. On the JavaScript target, the real fetch is async, so passthrough_sync only succeeds when a SyncTransport (typically a parity cassette) is installed; without one it returns the documented 501 sentinel telling the caller to use process_request_async.

pub fn passthrough_async(
  proxy: proxy_state.DraftProxy,
  request: proxy_state.Request,
) -> promise.Promise(
  #(proxy_state.Response, proxy_state.DraftProxy),
)

pub fn passthrough_sync(
  proxy: proxy_state.DraftProxy,
  request: proxy_state.Request,
) -> #(proxy_state.Response, proxy_state.DraftProxy)

pub fn passthrough_with_send_async(
  proxy: proxy_state.DraftProxy,
  request: proxy_state.Request,
  send: fn(request.Request(String)) -> promise.Promise(
    Result(commit.HttpOutcome, commit.CommitTransportError),
  ),
) -> promise.Promise(
  #(proxy_state.Response, proxy_state.DraftProxy),
)