test : added unit tests for github-rate-limit helpers

[tmdeveloper007]

Closes #2461.

Summary of What Has Been Done:
Added a dedicated vitest test file test/github-rate-limit.test.ts that pins the behaviour of every export in src/lib/github-rate-limit.ts: getGitHubRateLimitDetails, throwIfGitHubRateLimited, githubRateLimitResponse, and the GitHubRateLimitError class. Tests use a real Response constructed via new Response(null, { status, headers }) so the public surface area (including header parsing) is exercised end-to-end.

Changes Made:

• New file: test/github-rate-limit.test.ts (15 tests, all passing under npm test).
• Coverage for getGitHubRateLimitDetails:
  • 200 with x-ratelimit-remaining: 0 → null (status is the gate).
  • 404 → null.
  • 403 with x-ratelimit-remaining: 1 → null (still has budget).
  • 403 with x-ratelimit-remaining: 0 and a valid reset epoch → full GitHubRateLimitDetails with matching resetAt ISO string and resetAtEpoch.
  • 429 with the same conditions → same shape.
  • 403 with x-ratelimit-remaining: 0 but missing reset header → resetAt: null, generic message.
  • 403 with non-numeric reset header → resetAt: null, resetAtEpoch: null.
• Coverage for throwIfGitHubRateLimited:
  • 200 → no throw.
  • 403 with remaining=0 → throws GitHubRateLimitError whose details and message match getGitHubRateLimitDetails.
  • 403 with remaining=1 → no throw.
• Coverage for githubRateLimitResponse:
  • Returns null for plain Error, strings, null, undefined.
  • Returns a 429 Response with the right JSON body for a GitHubRateLimitError (both with and without a reset timestamp).

Impact it Made:
Locks the contract between GitHub's rate-limit signal and DevTrack's user-facing 429 response. Future refactors of the helper will surface a test failure rather than a silent security or UX regression. No production code is modified; the change is limited to one new test file.

[github-actions]

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