test : added unit tests for github-rate-limit.ts

[tmdeveloper007]

Closes #2654.

Summary of What Has Been Done:
Added comprehensive unit tests for the GitHub rate-limit detection helpers in src/lib/github-rate-limit.ts.

Changes Made:

• Added test/github-rate-limit.test.ts with 18 test cases across 3 describe blocks
• getGitHubRateLimitDetails(): 403 without quota exhaustion, 429 always triggers, 403 with exhausted quota, 403 with retry-after, missing reset header, non-numeric reset
• throwIfGitHubRateLimited(): non-throwing cases, throwing cases for 403 and 429
• githubRateLimitResponse(): non-rate-limit errors, null input, correct 429 response with rate limit body

Impact it Made:
Improves test coverage for GitHub API error handling used across metrics and sync features. Ensures rate-limit detection and graceful HTTP 429 responses are reliable.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[tmdeveloper007]

This PR was closed as stale. It has not received updates or merges within the past 2 days. If this is still needed, please reopen.