templates/worker-tinygo: make /echo work under wrangler dev

[shibukawa]

Summary

The worker-tinygo template's /echo endpoint fails under wrangler dev with:

TypeError: Body has already been used. It can only be used once. Use tee() first if you need to read it twice.

This PR fixes the template so it works out of the box, and documents why in a comment. Fixes #176.

Root cause

io.Copy(w, req.Body) triggers readableStreamToReadCloser.WriteTo's fast path in internal/jsutil/stream.go, which passes the request's raw ReadableStream through to the JS Response object via WriteRawJSBody.

This zero-copy passthrough is valid on production workerd, but miniflare routes every request through a proxy worker (miniflare/dist/src/workers/core/entry.worker.js), and that layer cannot relay a response whose body is the incoming request's own stream — await service.fetch(request) throws the error above.

This is an environment limitation, not a bug in this library. A plain JavaScript worker reproduces it under wrangler dev with no Go involved:

export default {
  async fetch(req) {
    return new Response(req.body); // same TypeError under wrangler dev
  },
};

(Related: cloudflare/workers-sdk#4373, another "streams work in production but not in miniflare" case.)

Since wrangler dev is the first thing every new user of the template runs, the template shouldn't rely on the passthrough. The worker-go template already buffers the body in /echo, which is why only the tinygo template was affected; this change aligns the two.

Testing

Verified with TinyGo 0.41.1 + wrangler 4.124.0 (and wrangler 3.109) on macOS:

$ curl -X POST -d "test message" http://localhost:8787/echo
# before: TypeError: Body has already been used. ... (HTTP 500)
# after:  test message

/hello unaffected.

[syumai]

Thanks!