Enable Node streams by default

[timneutkens]

What?

Defaults experimental.useNodeStreams to true while keeping the experimental config flag and explicit false opt-out available. Removes redundant dedicated node-stream CI jobs because the ordinary path exercises Node streams after this change.

Depends on the standalone test compatibility update in #94347, which intentionally remains outside the Node streams PR stack.

Why?

Node streams can be exercised through the standard Node.js App Router test path once they are the default, allowing a smaller rollout before the larger follow-up that removes the flag and obsolete plumbing.

CI exposed one necessary compatibility fix for this intermediate state: projects with a next.config.* received the new default after raw experimental-feature processing, leaving the runtime stream selector on the web implementation while compiled app code selected Node stream helpers.

How?

• Set defaultConfig.experimental.useNodeStreams to true.
• Synchronize __NEXT_USE_NODE_STREAMS from the fully resolved config, including cached and standalone resolved configs, so defaults and adapter modifications are reflected at runtime.
• Preserve the current public config/schema and explicit experimental: { useNodeStreams: false } opt-out; edge bundles continue to define the stream flag as false.
• Remove the dedicated node-stream test jobs and now-unused manifest; the affected streaming assertion compatibility changes are isolated in Delay Node Fizz piping until after render task #94347.

Verification

Run before extracting the prerequisite assertion changes into #94347:

• pnpm --filter=next types
• pnpm --filter=next build
• IS_WEBPACK_TEST=1 __NEXT_EXPERIMENTAL_STRICT_ROUTE_TYPES=true pnpm test-start-webpack test/e2e/app-dir/ppr-root-param-fallback/ppr-root-param-fallback.test.ts
• IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 __NEXT_EXPERIMENTAL_STRICT_ROUTE_TYPES=true pnpm test-start-turbo test/e2e/app-dir/ppr-root-param-fallback/ppr-root-param-fallback.test.ts
• IS_WEBPACK_TEST=1 __NEXT_EXPERIMENTAL_STRICT_ROUTE_TYPES=true pnpm test-start-webpack test/e2e/app-dir/use-server-inserted-html/use-server-inserted-html.test.ts
• pnpm test-dev-webpack test/e2e/app-dir/ppr-root-param-fallback/ppr-root-param-fallback.test.ts test/e2e/app-dir/use-server-inserted-html/use-server-inserted-html.test.ts
• pnpm test-dev-turbo test/e2e/app-dir/ppr-root-param-fallback/ppr-root-param-fallback.test.ts test/e2e/app-dir/use-server-inserted-html/use-server-inserted-html.test.ts
• pnpm test-start-webpack test/production/app-dir/use-node-streams-env-precedence/use-node-streams-env-precedence.test.ts
• pnpm test-start-webpack test/e2e/app-dir/app/standalone.test.ts

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[unstubbable]

Why do we need any test changes in this PR? Shouldn't those tests already have been verified with Node streams in the specific jobs that this PR deletes?

[vercel]

Additional Suggestion:

Leaving useNodeStreams enabled by default while reverting the two test files' node-stream assertions makes those e2e tests fail under the now-default node streams.