Add RBS validation to CI pipeline

Add RBS type signature validation to the lint-js-and-ruby.yml workflow.
This will catch type signature regressions automatically before merge.

The validation step runs after RuboCop, using the same command as the
existing rake task: bundle exec rbs -I sig validate

Fixes #1952

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

.github/workflows/lint-js-and-ruby.yml

@@ -89,6 +89,8 @@ jobs:
         run: bundle check --path=vendor/bundle || bundle _2.5.9_ install --path=vendor/bundle --jobs=4 --retry=3
       - name: Lint Ruby
         run: bundle exec rubocop
+      - name: Validate RBS type signatures
+        run: bundle exec rbs -I sig validate
       - name: Install Node modules with Yarn for dummy app
         run: cd spec/dummy && yarn install --no-progress --no-emoji --frozen-lockfile
       - name: Save dummy app ruby gems to cache