Follow-up from #3481 (now closed). The OSS/SSR dummy (react_on_rails/spec/dummy) and the Pro rspack-RSC runtime gate now exercise Rspack in CI, but react_on_rails_pro/spec/execjs-compatible-dummy still builds only with webpack.

Scope

• Add an Rspack build/test leg for react_on_rails_pro/spec/execjs-compatible-dummy (e.g. SHAKAPACKER_ASSETS_BUNDLER=rspack), mirroring the matrix approach used in dummy-app-bundler-tests.yml / integration-tests.yml.
• Keep the existing webpack leg green (breadth, not replacement).

Notes

• This is a minimal execjs fixture, so the work is small; low priority.
• Confirm shakapacker-rspack and the required @rspack/* peers are present in that dummy's package.json first.

Priority: low (P3).