fix: resolve timeout and rate limiting in github repo fetching (#153)

[Rajen5145]

Description

Summary of Changes

• Error Handling & Architecture Alignment: Refactored fetchUserRepos to use the project's internal httpRequest utility and assertOk validation, preserving proper mapping for GitHub error statuses (NOT_FOUND, RATE_LIMIT, API_DOWN, and API_ERROR).
• Optimized Asynchronous Flow: Streamlined data resolution in getGitHubUserData to execute network requests concurrently via Promise.all, preventing unnecessary serial bottlenecks and reducing overall API latency.
• Pagination Integrity: Maintained complete multi-page repository fetching support up to the final page while eliminating unhandled infinite-loop failure points that trigger serverless execution timeouts.
• Testing & Verification: Ran full test suites locally via npm test—all 19 test suites and 322 unit tests passed with zero regressions.

Closes #153

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