Fix dynamic App Store auth endpoint discovery

[tosbaha]

It seems the App Store auth endpoint doesn't work. I tested end-to-end, and it worked for my old account, which doesn't have 2FA.

Disclaimer: Most of the code is written by AI, and I am putting it here so that maybe it can help you to solve this issue faster.

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Summary by cubic

Fixes dynamic App Store auth endpoint discovery and normalization to keep login working when Apple changes URLs. We now default to the native /fast/ endpoint and retry login when a new endpoint is found in responses.

• Bug Fixes
  • Read authenticateAccount from the bag root or urlBag, normalize it, and fallback to https://auth.itunes.apple.com/auth/v1/native/fast/.
  • During login, normalize the provided endpoint; on decode errors, extract an auth URL from the response and retry with the discovered endpoint.
  • Added ResponseDecodeError in pkg/http to capture status, content type, a trimmed body, and extracted URLs for discovery.
  • Introduced endpoint helpers in pkg/appstore (normalizeAuthEndpoint, etc.) and added tests covering bag, login retry, and normalization.

^{Written for commit a98f833. Summary will update on new commits.}

[ZomkaDEV]

What are the difference between this PR and this one? #490

[tosbaha]

@ZomkaDEV I first tried their PR and it didn't work for me. I asked AI to fix it and my version worked.

[pmalecka]

This one works well, thanks!

[viktorkuts]

Can vouch for this PR, worked for me as well, lifesaver, many thanks!