fix: assorted bug fixes (close #588)

[andriytyurnikov]

First slice of #607, per your request to break it up. Four independent bug fixes, each in its own commit. No version bumps, no CI changes.

Fixes

vite-plugin-ruby: stop forcing sourcemap=true in production (closes #588)

The plugin was setting build.sourcemap = !isLocal, which inverted Vite's safe default:

• production / staging → sourcemap=true (publishes .js.map publicly, exposing source)
• development / test → sourcemap=false (breaks debugging where you'd actually want it)

Drop the override entirely. Vite's default (false) now applies in every mode; consumers opt in via their own build.sourcemap config when they want sourcemaps (e.g. uploaded to Sentry).

Tests updated:

• vite-plugin-ruby/tests/index.spec.ts — assert sourcemap is no longer set
• vite-plugin-rails/tests/build.spec.ts — drop .map entries from expected file list

examples/rails: gate engine alias on env var

The example used a TypeScript non-null assertion on process.env.ADMINISTRATOR_ASSETS_PATH, which is unset at build time when no Rails engine is wired up — leaving [undefined] in server.fs.allow and an alias resolving to "undefined/...". Vite ≤7 silently tolerated this; Vite 8 strictly path-resolves server.fs.allow and crashes.

Make both the alias and the fs.allow entry conditional on the env var actually being present.

vite-plugin-ruby: return after res.end() in dev /index.html 404 fallback

Without the explicit return, control falls through to next() after the response has already been ended, allowing downstream middleware to write to a closed response.

vite_ruby: guard dev_server_running? cache with a Mutex

The prior implementation read @running and wrote @running_checked_at without synchronization, so a cold concurrent call from a multi-threaded server (e.g. Puma) could read @running after it was set but before @running_checked_at was, and the next call's Time.now - @running_checked_at could raise NoMethodError on nil.

Wrap the check-or-refresh in @running_mutex.synchronize so the cache read and write are atomic.

Test plan

• pnpm -C vite-plugin-ruby test --run passes
• pnpm -C vite-plugin-rails test --run passes
• Local examples/rails vite build succeeds with and without ADMINISTRATOR_ASSETS_PATH set
• CI matrix passes