[v7] Run buildEnd after preview prerendering

[brophdawg11]

Summary

• Backport Run buildEnd after prerendering #15211 so the future-flagged prerender plugin runs buildEnd after prerender output is written
• Keep legacy prerender cleanup behavior unchanged outside the Vite Environment API path
• Add focused integration coverage for unstable_previewServerPrerendering

Testing

• pnpm run --filter @react-router/dev build
• pnpm test:integration:run integration/vite-prerender-test.ts --project chromium -g "Runs buildEnd after prerendering is complete"

[github-actions]

Preview Build Available

Preview builds have been created for this PR. You can install react-router using:

pnpm install "remix-run/react-router#preview/pr-15212&path:packages/react-router"

And/or install other packages via:

pnpm install "remix-run/react-router#preview/pr-15212&path:packages/react-router-dev"
pnpm install "remix-run/react-router#preview/pr-15212&path:packages/react-router-express"
pnpm install "remix-run/react-router#preview/pr-15212&path:packages/react-router-node"
pnpm install "remix-run/react-router#preview/pr-15212&path:packages/react-router-serve"

These preview builds will be updated automatically as you push new commits.