fix(vitest-pool-workers): responses from fetchMock should have immutable headers

[edmundhung]

Fixes #7468.

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• Tests
  • TODO (before merge)
  • Tests included
  • Tests not necessary because:
• Wrangler E2E Tests CI Job required? (Use "e2e" label or ask maintainer to run separately)
  • I don't know
  • Required
  • Not required because: no impact outside of vitest-pool-workers
• Public documentation
  • TODO (before merge)
  • Cloudflare docs PR(s):
  • Documentation not necessary because: bugfix

[changeset-bot]

🦋 Changeset detected

Latest commit: e54d234

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

This PR includes changesets to release 1 package

Name	Type
@cloudflare/vitest-pool-workers	Patch

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[github-actions]

A wrangler prerelease is available for testing. You can install this latest build in your project with:

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cloudflare-workers-bindings-extension:

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@cloudflare/kv-asset-handler:

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miniflare:

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npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/13550691793/npm-package-cloudflare-vitest-pool-workers-8254

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---

[email protected] includes the following runtime dependencies:

Package	Constraint	Resolved
miniflare	workspace:*	3.20250214.1
workerd	1.20250214.0	1.20250214.0
workerd --version	1.20250214.0	2025-02-14

Please ensure constraints are pinned, and miniflare/workerd minor versions match.