fix: vinext start for Cloudflare App Router worker exports

[JaredStowell]

Fix vinext start for App Router production builds emitted through @cloudflare/vite-plugin.
Reported here: #459

Cloudflare App Router builds can default-export a Worker-style object with fetch() instead of a plain request handler function. vinext start assumed the default export was always callable, which caused startup to fail with:

[vinext] RSC entry does not export a default handler function

This change teaches the production server to accept both supported App Router entry shapes:

• default (request) => Response
• default { fetch(request, env, ctx) }

It also keeps an explicit error for unsupported export shapes.

Changes

• add App Router entry resolution in the prod server
• support Worker-style default exports during vinext start
• provide a minimal Node-side execution context so ctx.waitUntil() does not crash
• add regression tests for:
  • Worker-style App Router default exports
  • clear failure on invalid export shapes

Verification

Ran:

• pnpm test tests/app-router.test.ts
• pnpm run fmt
• pnpm run typecheck

Also verified manually with a real Cloudflare App Router build:

• built examples/benchmarks
• started it with node ../../packages/vinext/dist/cli.js start --port 4311
• confirmed GET / returned 200 OK

[pkg-pr-new]

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npm i https://pkg.pr.new/vinext@460

commit: 497e78a