fix: start transferred MessagePort in fetch-sync worker (#9198)

* fix: start transferred MessagePort in fetch-sync worker

This was causing running Miniflare in Bun tests to hang indefinitely.

* add changeset

---------

Co-authored-by: Peter Bacon Darwin <[email protected]>

Author: kylecarbs

.changeset/stupid-files-jog.md

@@ -0,0 +1,9 @@
+---
+"miniflare": patch
+---
+
+fix: ensure the fetch proxy message port is started
+
+While Node.js will start the message port automatically when a `message` event listener is added,
+this diverges from the standard Web API for message ports, which require you to explicitly start
+listening on the port.

packages/miniflare/src/plugins/core/proxy/fetch-sync.ts

@@ -82,6 +82,8 @@ port.addEventListener("message", async (event) => {
   Atomics.store(notifyHandle, /* index */ 0, /* value */ 1);
   Atomics.notify(notifyHandle, /* index */ 0);
 });
+
+port.start();
 `;
 
 // Ideally we would just have a single, shared `unref()`ed `Worker`, and an