fix: send required GitHub API headers

[Quantumlyy]

Summary

• Add required GitHub REST API headers, including User-Agent, to OAuth user detail requests.
• Log sanitized GitHub /user failure details to make future OAuth callback 502s diagnosable.
• Cover the OAuth callback path with a test that verifies the GitHub API request headers.

Verification

• pnpm test in apps/gateway
• pnpm build in apps/gateway

---

Created with Constructor

[Copilot]

Pull request overview

This PR fixes the gateway OAuth callback path by adding GitHub REST API headers to user-detail requests and adding coverage to verify those headers are sent.

Changes:

• Adds shared GitHub API headers for /user and /user/emails fetches.
• Logs bounded GitHub /user failure details for diagnosability.
• Adds an OAuth callback test that verifies GitHub API request headers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
apps/gateway/src/auth.ts	Adds GitHub API headers and failure logging in the OAuth callback user fetch path.
apps/gateway/test/index.spec.ts	Adds a mock OAuth callback test covering GitHub API headers.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.