[quick-edit] Use X-Forwarded-Host header for extension authority when behind a proxy

[petebacondarwin]

When Quick Edit is accessed through a proxy (e.g., on devprod.cloudflare.dev), the browser sees the proxy's host, but the Worker receives its own host in the URL. The additionalBuiltinExtensions config tells VS Code where to load extensions from, so we need to use the proxy's host for the browser to load them correctly.

This change:

• Reads the X-Forwarded-Host header from incoming requests
• Validates it matches *.devprod.cloudflare.dev for security
• Uses it as the authority for extension URLs when valid, falling back to url.host otherwise

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because: This is a small, self-contained change that only affects the authority used for extension loading when behind a validated proxy
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: This is an internal implementation detail for proxy support

---

[changeset-bot]

🦋 Changeset detected

Latest commit: 87685a8

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

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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commit: 87685a8