test : added unit tests for ScanRateLimiter helpers in rate_limiter

[tmdeveloper007]

Summary

Added 17 unit tests for helper functions in ScanRateLimiter from backend/secuscan/rate_limiter.py.

Changes Made

• Created testing/backend/unit/test_rate_limiter_scan_rate_limiter.py with 17 tests
• Tests cover:
  • _get_client_ip: proxy headers (X-Forwarded-For), direct clients, missing headers
  • _build_error_response: formatting of 429 JSON responses, retry-after headers, X-Request-ID
  • _check_burst_limit: burst window tracking, first-request initialization, limit enforcement
  • Rate limiter state isolation between test runs via mock cleanup

Impact

All 17 tests pass. The rate limiter module had zero unit test coverage before this PR.

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This task is being handled by tmdeveloper007 as part of the GSSOC Auto-PR cron.

[utksh1]

Thanks for adding test coverage! This PR needs a rebase with main to pass CI. Please run:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

Once rebased and CI passes, I'll merge immediately.

[tmdeveloper007]

Thanks! I have rebased the branch on the latest upstream/main and pushed. The PR should now be up to date. Please re-review when you have a chance.

[utksh1]

Reviewed the updated test submission. The coverage is focused and distinct from the duplicate PRs already closed. Backend scope is acceptable; merge remains subject to a current branch and the repository-required frontend security audit.