fix: fall back to legacy Store authentication

[majd]

This pull request improves the App Store login flow by adding robust handling for legacy authentication fallbacks and better error reporting when Apple returns unexpected responses. It introduces a new error type to capture non-XML or empty responses, ensures the correct request payload is preserved during redirects, and updates tests to cover these scenarios.

App Store login improvements:

• Added fallback logic in appstore_login.go to retry login with the legacy authentication endpoint (legacyAuthenticateEndpoint) when the native endpoint returns specific HTTP errors or empty responses. [1] [2] [3]
• Ensured that when following authentication redirects (such as to Store pods), the original XML plist request body and its attempt value are preserved. [1] [2]

Error handling enhancements:

• Introduced a new UnexpectedResponseError type in client.go to capture HTTP status and response snippets when Apple returns an empty or non-plist body.
• Updated the XML response handler to return this error type and to preserve redirect responses and their headers for callers.

Test coverage:

• Added and updated tests in appstore_login_test.go and client_test.go to verify fallback to legacy authentication, correct handling of redirects, and preservation of request bodies and headers. [1] [2] [3]

[majd]

@Horskin Can you fix the linting issues so we can get this merged?

[tux-mind]

bboy910225#1

[iossp-vn]

Idk why, but whether I use an ID with or without 2FA, I still can't log in. When I tried with an ID (no 2FA), it still asked me for a 2FA code.