shopify_draft_proxy/proxy/upstream_dispatch

Driver that fetches a GraphQL request upstream when the proxy is in LiveHybrid mode. Mirrors the commit-driver split:

• fetch_sync/4 — Erlang. Synchronous; returns a Result directly.
• fetch_async/4 — JavaScript. Returns a Promise(Result(...)).

Both share the same send-shaped seam (built from upstream_client.send_*) so tests can inject a fake transport without dragging gleam_httpc/gleam_fetch into the assertion shape.

pub fn fetch_async(
  origin: String,
  path: String,
  body: String,
  inbound_headers: dict.Dict(String, String),
  send: fn(request.Request(String)) -> promise.Promise(
    Result(commit.HttpOutcome, commit.CommitTransportError),
  ),
) -> promise.Promise(
  Result(commit.HttpOutcome, commit.CommitTransportError),
)