feat: add loadServerModule API for SSR-only TypeScript modules

[thetutlage]

Summary

• Adds vite.loadServerModule(entry, opts?) — a first-class API for loading server-side TypeScript modules through Vite in both dev and build mode.
• Adds serverEntrypoints plugin option that bundles declared entries into the SSR environment (one bundle per entry, hash-free, with manifest).
• Moves process.env.NODE_ENV = 'production' from the Inertia Vite plugin to @adonisjs/vite's build hook — it's a generic concern, not Inertia-specific.

Why

Today, packages that need to execute TypeScript on the server (e.g. Inertia SSR, React-email templates, MDX renderers) each roll their own Vite bundling + module-runner plumbing. This PR consolidates that into a single API so any consumer can load Vite-processed modules from Node without owning a ModuleRunner themselves.

Design

• Roles, not helpers. Logic lives in src/server_modules/ split into ServerModuleLoader (orchestrator), DevModuleRunner (runner host with restart-detect), and BundledModuleResolver (manifest read + cached prod imports).
• Manifest as source of truth in prod. The SSR environment emits a manifest mirroring the client pattern; loadServerModule resolves entry → bundle file through it. No string normalization, no extension stripping — entry strings are passed through verbatim, just like client entrypoints.
• Override-safe config. Every environments.ssr.build.* field falls back to user/plugin-supplied values via ?? guards, so other plugins contributing to the SSR environment cooperate rather than collide.
• Type-safe entries. Apps and packages augment the exported ServerModuleMap interface to associate entrypoint paths with their module shapes.
• fresh opt-in. loadServerModule(entry, { fresh: true }) clears the runner cache before importing. Default is to trust HMR for invalidation — no per-render clearCache waste.

Build hook changes

• Sets NODE_ENV=production before createBuilder so framework plugins emit production paths under programmatic builds (see import_jsx_dev_runtime.jsxDEV is not a function after upgrade to 1.7.0 in Docker Container remix-run/remix#4081).
• Forces multi-environment builder mode (useLegacyBuilder = false) so Vite's default buildApp builds every declared environment when no plugin contributes its own.

Migration path for Inertia (separate PR)

1. ServerRenderer.render() → vite.loadServerModule(config.ssr.entrypoint).
2. Drop the clearCache call (or pass fresh: true if a regression is observed).
3. Delete @adonisjs/inertia/src/client/vite.ts (the Inertia Vite plugin) — its responsibilities are now covered by serverEntrypoints and the build hook.
4. Document upgrade: replace inertia() plugin in vite.config.ts with serverEntrypoints: ['inertia/ssr.tsx'] on adonisjs().

Test plan

• 11 new tests in tests/backend/server_modules.spec.ts covering:
  • dev path: TS/JSX import via runner, runner reuse across calls, fresh: true re-evaluates
  • prod path: manifest-driven import, per-entry import cache, missing-entry error, missing-manifest error
  • configHook: ssr environment absent when serverEntrypoints is empty, present when non-empty, defers to user-supplied fields
• Full test suite green (113 passed, 1 skipped)
• tsc --noEmit clean