Add RBS validation to CI pipeline

[justin808]

Summary

Add bundle exec rbs -I sig validate to the CI pipeline to catch type signature regressions automatically.

Motivation

Currently, RBS type signatures are validated manually via rake task. Adding this to CI would:

• Prevent type signature drift
• Catch type errors before merge
• Ensure signatures stay in sync with implementation changes
• Provide confidence that type signatures remain valid

Implementation

Add to CI workflow (likely in .github/workflows/):

- name: Validate RBS type signatures
  run: bundle exec rbs -I sig validate

Or add to existing lint/check step:

- name: Lint and type check
  run: |
    bundle exec rubocop
    bundle exec rbs -I sig validate

Existing Infrastructure

• Rake task already exists: rake rbs:validate
• Located in rakelib/rbs.rake
• Currently passes: ✓ RBS validation passed

Related

Follow-up to PR #1945 - post-merge code review feedback

Priority

High - This would prevent regressions and is a one-time setup cost with ongoing benefits.

[justin808]

This issue was already resolved by PR #1950, which added comprehensive RBS validation to CI.

The implementation in PR #1950 is more complete than originally requested:

• Added rake rbs:validate to the main lint workflow (.github/workflows/lint-js-and-ruby.yml:108)
• Added RBS validation to Pro gem lint workflow
• Added Steep static type checker (currently disabled pending fixes)
• Added runtime RBS checking during tests

The RBS validation now runs automatically in CI and will catch type signature regressions before merge.