[Website] Fix redirect loop on Safari by awaiting cookie store population and re-scoping redirect URLs

[mho22]

Motivation for the change, related issues

#3361 introduced streamed PHP responses in the service worker. This changed the cookie store population from synchronous (awaited via PHPResponse.fromStreamedResponse()) to a floating .then() that could resolve after the next request arrived. This caused playground_auto_login to redirect infinitely on Safari because the playground_auto_login_already_happened cookie was never stored in time. Response.redirect() drops Set-Cookie headers, and the internal cookie store was the only mechanism preserving them across requests.

A secondary issue was that Response.redirect(new URL('/wp-admin/', scopedUrl)) resolves absolute paths against the origin, stripping the /scope:…/ prefix.

cc @bgrgicak

Implementation details

• php-request-handler.ts: Replace the floating .then() with await response.headers before returning from requestStreamed(), ensuring the cookie store is populated before the next request can arrive. Headers are available as soon as PHP flushes them, well before the body finishes streaming, so this does not hurt throughput.
• utils.ts: Re-scope the Location URL before calling Response.redirect(). If the resolved URL lacks the /scope:…/ prefix, add it back using setURLScope.

Testing Instructions (or ideally a Blueprint)

run npm run dev and go to http://127.0.0.1:5400/website-server/ on safari

It will crash on trunk and run correctly from this branch.

[Copilot]

Pull request overview

Adjusts landing-page redirection to use relative paths so Safari’s service-worker Response.redirect() workaround preserves the scoped URL prefix during resolution.

Changes:

• Adds detailed rationale/docs for why relative landing-page paths are required for scoped service-worker routing.
• Replaces pathToInternalUrl() usage with a simple absolute-to-relative path rewrite (/foo → ./foo) before redirecting via the intermediate handler.

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[brandonpayton]

This looks great. Thank you, @mho22!