Read the Apple service key from the sign out redirect (fixes #161)

[sebsto]

Fixes #161.

Problem

xcodeinstall authenticate fails at the very first step with unableToRetrieveAppleServiceKey. Apple removed the olympus config endpoint in September 2026:

GET https://appstoreconnect.apple.com/olympus/v1/app/config?hostname=itunesconnect.apple.com
-> 404 Not Found

getAppleServicekey() relied on that endpoint to fetch the widget key (authServiceKey) required by every subsequent auth call, so login dies before it can start. Same break as fastlane/fastlane#30199; the upstream fix is fastlane/fastlane#30206.

Fix

The widget key is a public, static front-end identifier, not a credential. Apple's own App Store Connect front end now reads it from the sign-out redirect:

GET https://appstoreconnect.apple.com/logout
-> 302  Location: https://idmsa.apple.com/appleauth/signout?widgetKey=<key>&asop=destroy-session&...

getAppleServicekey() now tries that source first and falls back to the olympus endpoint (kept in case Apple restores it):

• fetchServiceKeyFromSignout() reads widgetKey out of the /logout 302 Location header, returning nil on any expected miss so the caller falls through to olympus rather than making this a new single point of failure.
• signoutRedirectLocation() issues the request over a dedicated ephemeral URLSession with a redirect-suppressing delegate and no cookie storage. This is deliberate: the redirect target carries asop=destroy-session, so following it (or sending the session cookies) would actually sign the session out. We stop at the 302 and read only the header.
• fetchServiceKeyFromOlympus() is the previous behaviour, unchanged, now the fallback.

NoRedirectDelegate is a stateless, Sendable delegate returning nil from willPerformHTTPRedirection.

Tests

New cases mirror the upstream fix; the seam is stubbed via a StubbedSignoutAuthenticator test subclass so no live network is hit:

• reads the key out of the sign-out Location redirect;
• falls back to olympus when the redirect carries no widgetKey;
• falls back to olympus when the sign-out request fails outright;
• the existing olympus 200/500 and no-service-key flow tests still pass (olympus is the fallback path now).

Built and tested locally on macOS with Xcode 26.6: swift build clean, swift test green (207 tests, 19 suites).