Add official guide or helper for integration/system testing with RSC and the Node renderer

[justin808]

Context

While building the Hacker News demo at https://github.com/shakacode/react_on_rails-hacker-news-app, getting integration/system tests stable with RSC + the Node renderer took more manual setup than expected.

We ended up needing a fairly specific test harness:

• set renderer-related env vars before Rails boot
• compile test assets ahead of time
• start a dedicated renderer process for the test environment
• isolate or clear renderer bundle caches
• avoid unsafe parallelization in certain paths
• use a fake Hacker News API to keep tests deterministic

Problem

There is not yet a clear first-class recipe for testing this stack in a normal Rails app, especially for Capybara/system tests and end-to-end integration tests.

That leads to failures that look like timeouts, missing assets, or renderer instability instead of obviously being a setup problem.

Requested improvement

Please add official guidance or a helper for integration/system testing with RSC and the Node renderer.

Useful coverage would include:

• when to compile assets for test
• how to boot/shutdown the renderer in test
• bundle-cache isolation guidance
• caveats for parallel test execution
• a concrete Capybara/system test example
• guidance on stubbing external APIs while still exercising the RSC path

Related

Issue #3076 touched CI setup and some RSC integration pitfalls. This issue is narrower and asks for a first-class testing recipe/helper for app authors.