Fix 429 rate limit handling and context key type mismatch in login #480

[rudSarkar]

This pull request introduces minor improvements for better code clarity and error handling. The most important changes are:

Error handling improvements:

• Added explicit handling for HTTP 429 (Too Many Requests) responses in pkg/http/client.go, returning a clear error message when rate limited by Apple.

Code clarity:

• Updated the loginCmd function in cmd/auth.go to use the interactiveKey constant for context value access, improving type safety and maintainability.

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Summary by cubic

Adds explicit handling for HTTP 429 (Too Many Requests) from Apple with a clear error, and fixes the login context key mismatch by using the interactiveKey constant. This prevents confusing failures under rate limiting and improves type safety.

^{Written for commit 88922f5. Summary will update on new commits. Review in cubic}