Improve Pages API route parity

[southpolesteve]

Extracts a focused slice from #922 into a reviewable PR.

This PR improves Pages API route compatibility for the Next.js deploy-suite work without pulling in the broader Pages Router rendering/client-navigation changes.

Includes:

• route-level config.api.bodyParser.sizeLimit support
• config.api.bodyParser = false raw request stream support
• piping raw request streams into responses without forcing res.end()
• res.revalidate() callback plumbing
• best-effort Pages API runtime = "edge" / experimental-edge handling via NextRequest, with a warning that vinext does not implement true Edge Runtime isolation
• an AsyncLocalStorage edge-runtime global helper for parity with edge API fixtures

Verification run locally:

• vp test run tests/pages-api-route.test.ts
• vp check packages/vinext/src/server/pages-api-route.ts packages/vinext/src/server/pages-node-compat.ts packages/vinext/src/server/edge-runtime-globals.ts tests/pages-api-route.test.ts
• vp run vinext#build

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vinext@1205

commit: a6881d5

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
benchmarks	preview	production
hackernews	preview	production	original