test: abort with a clear message when built assets are missing

[costajohnt]

What github issue is this PR for, if any?

Resolves #7100

What changed, and why?

On a fresh checkout, app/assets/builds/ is empty (it is gitignored and only populated by npm run build and npm run build:css). manifest.js links that directory, and link_directory only declares files that exist, so a layout that links tailwind.css raises AssetNotPrecompiledError with advice to edit the manifest, which is already correct. A missing application.js fails with "not present in the asset pipeline".

• spec/rails_helper.rb: abort at boot when any of the three bundles the layouts link (application.js, all_casa_admin.js, tailwind.css) is missing, naming the commands that build them. Sits next to the existing pending-migration abort and follows the same pattern.
• README.md: note under Running Tests that bin/rails spec builds the bundles first (it runs spec:prepare, which jsbundling/cssbundling hook into), while a bare bundle exec rspec does not.

Two things from the issue that did not hold up:

• chore: disable on-demand asset compilation and normalize Gemfile.lock platforms (#7040) #7071 only changed config/environments/production.rb; the test environment still has config.assets.compile = true. The cause is the missing build output, not on-demand compilation being off.
• npm run build:css works from a clean npm ci here (arm64 macOS, Node 24.18, npm 11): @tailwindcss/cli and the tailwindcss bin are installed and the CSS is written. So I left package.json alone.

bin/setup already runs both builds since #7051, so that part of the issue was done.

How is this tested? (please write rspec and jest tests!) 💖💪

No spec: the guard runs while rails_helper loads and aborts the process, and the repo has no precedent for subprocess specs. Checked by hand:

• bundle exec rspec spec/requests/case_court_reports_spec.rb:14:28:37 with the bundles built: 3 examples, 0 failures.
• With app/assets/builds/tailwind.css removed, bundle exec rspec spec/requests/case_court_reports_spec.rb:14 exits 1 and prints Missing built assets: tailwind.css. Run \npm run build` and `npm run build:css`, or use `bin/rails spec`, which builds them.` before anything else.
• With tailwind.css removed, bin/rails spec SPEC=... rebuilds it and passes, which is why the README wording distinguishes the two.
• bundle exec standardrb spec/rails_helper.rb clean.

CI runs rails assets:precompile before rspec, so the guard cannot fire there.

Screenshots please :)

Terminal output with tailwind.css removed:

$ bundle exec rspec spec/requests/case_court_reports_spec.rb:14
Missing built assets: tailwind.css. Run `npm run build` and `npm run build:css`, or use `bin/rails spec`, which builds them.