Port GSA authentication fixes (AltSign PRs #50/#51) to Windows

[shanegillio]

This ports the GSA sign-in fixes from rileytestut/AltSign#50 and rileytestut/AltSign#51 to AltServer for Windows. The macOS versions were confirmed working against the recent GSA 503 / error-3840 sign-in failures; the Windows code has the same blind spots, so it presumably fails the same way.

Changes (all in AltSign/AppleAPI+Authentication.cpp)

• User agent: replaces the 2019-era akd/1.0 CFNetwork/978.0.7 Darwin/18.7.0 string with AuthKit/1 (Macintosh; OS X 26.5.2) (com.apple.dt.Xcode/26.0), matching AltSign PR Port GSA authentication fixes (AltSign PRs #50/#51) to Windows #51.
• Retry on 5xx: SendAuthenticationRequest now retries up to 5 times with 1/2/4/8s backoff. Each attempt constructs a fresh http_client — the cpprestsdk equivalent of the ephemeral URLSession in PR Hello, after upgrading to version 26.4 and installing it via a self-signed installer, the app crashes. What could be the reason? #50 — because Apple's GSA edge pins a keep-alive connection to a backend node, and once that node starts 5xx-ing, retries on the same connection never recover.
• Descriptive errors: when Apple returns a non-plist body (typically an HTML error page), the error now surfaces the HTTP status, Content-Type, and a body snippet via LocalizedAPIError instead of a bare APIErrorCode::InvalidResponse.
• Trusted-device 2FA: the verify-code response goes through the same validation.

Notes for review

• Untested on Windows — I don't have a Windows build environment, so this compiles-by-inspection only and needs a build + sign-in test on a real setup. Submitting for review so it can be compiled and tested on the correct platform.
• Based on the 1.7.4 branch (hence the base). Happy to rebase elsewhere if there's a better target.
• One consideration: e91915d switched X-Mme-Client-Info to a native Windows AuthKit client ID, while the new User-Agent identifies as Xcode on a Mac. If the patched build still hits 503s, that mismatch is the first thing worth investigating.

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