Validate Fetch options in the target edge runtime

Do not treat a Node fetch mock as compatibility evidence for Cloudflare Workers. A mock may accept redirect: "error" while Workerd rejects the option synchronously, before any network request. For fail-closed subrequests—especially those carrying credentials—use a mode the target runtime accepts, such as redirect: "manual", then reject every 3xx response explicitly and preserve exact-origin checks. Add a contained Miniflare/Workerd contract test alongside unit tests. This is runtime- and version-specific; documentation and implementation may diverge, so pin tested versions and rerun after upgrades. Manual mode only prevents automatic following; application code must enforce rejection.

Applies when: Worker code relies on Fetch options for security-sensitive redirect handling. Keywords: Cloudflare Workers Workerd Miniflare fetch redirect manual TypeError unit mock runtime