Start: public helper to call one server function without rendering a route

[alexander-zuev]

Problem. A test that calls one server function through the real Start handler has no public client. createClientRpc reads the private tanstack-start:start-storage-context before it sends anything. serverFnFetcher, which builds the HTTP request, is not exported. createStartHandler does not help: its callback runs only after the route loads, a non-HTML request returns 406, and a /_serverFn/ request never calls the callback. Reaching the public handler means copying an untyped wire protocol (x-tsr-serverFn, seroval, { result, error, context }), which a Start upgrade can break with no type error.

Request. Export a helper that calls one server function through a fetch handler, without rendering a route and without building that private context:

const response = await createServerFnFetch(handler.fetch)(getPriceRevision, {
  data: { id },
  method: "GET",
  headers,
})

It should build and parse the request, run the app's real handler, and not call getRouter() or router.load().

Context. @tanstack/react-start 1.168.60. Related: discussion #2701 and issue #7507. This is the HTTP side of that gap.