Fail early when createTestHarness() cannot start its proxy under Bun

[dario-piotrowicz]

Fixes #15717

When Bun does not deliver the proxy control request required by Miniflare, createTestHarness() now throws an actionable startup error. This prevents the process from remaining idle while every request to the test harness hangs indefinitely.

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: better error handling

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[changeset-bot]

🦋 Changeset detected

Latest commit: 0d71c0e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
wrangler	Patch
@cloudflare/vite-plugin	Patch
@cloudflare/vitest-plugin	Patch

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[ask-bonk]

LGTM

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commit: 0d71c0e

[oddharsh]

I put the change from my issue comment up as #15846, targeting this branch so it reads as a diff on top of your commit. It keeps your detection and the timeout helper, and moves where they fire:

• listen() and getWorker().fetch() work under Bun again.
• server.fetch() throws your UserError, reworded to point at getWorker().fetch().
• The ProxyWorker answers requests to the listen() URL with a 503 naming undici shim: Pool lacks dispatch()/close()/destroy(), breaking miniflare + workerd (wrangler dev, @cloudflare/vite-plugin) oven-sh/bun#39247 once a control request arrives without its payload, so fetch(url) fails loudly too.

The PR has a test that boots a real ProxyWorker under Node and sends the control request the way Bun does. Against this branch's ProxyWorker it times out, which reproduces the hang without Bun. The red macOS check there is the miniflare R2 ECONNRESET flake (s3.spec.ts), which passes 71/71 locally, and the PR touches nothing in miniflare.

Merge it in or take the idea, whichever suits you.