fix(ci): fix Ruby integration test failures

[CalvinAllen]

Summary

• Use 'all' source selection in rbenv migration tests (macOS and Ubuntu) to handle cases where system Ruby is detected before rbenv
• Handle GitHub API rate limiting (HTTP 403/429) gracefully in Ruby integration test version verification step

Problem

The integration tests were failing due to:

1. rbenv migration tests: On GitHub Actions runners, system Ruby (via Homebrew on macOS) was being detected as the first source instead of rbenv. Using "1\n0\nn\n" was selecting the system Ruby (3.3.10) instead of the rbenv-installed version (3.3.6).
2. Ruby integration test: GitHub API rate limiting (HTTP 403) was causing list-all ruby to fail, which blocked the entire test run.

Solution

1. Changed rbenv migration tests to use "all\n0\nn\n" to select all sources, similar to the uru migration fix in fix(ci): enable all migrations and fix uru version selection #157
2. Added rate limit detection in Ruby integration tests - if API returns 403/429, skip version verification and proceed with install (which will fail anyway if version doesn't exist)

Test plan

• npm run check passes (format, lint, tests)
• Re-run integration tests to verify fixes work