App Router: `reactMaxHeadersLength` config not respected

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

The reactMaxHeadersLength config option (which controls how much React inlines into HTTP headers like Link preload hints) is not honored — the expected header that should be set when this option is configured is always null.

With `reactMaxHeadersLength` configured, expected Link preload header set; received null

Estimated Impact

~3 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/react-max-headers-length/react-max-headers-length.test.ts (3 failures)

Recommendation

1. Reproduce first in vinext's own test suite. Configure reactMaxHeadersLength: 16000 and assert the expected Link header is emitted.

2. Forward the option into the React renderer. Pass reactMaxHeadersLength to renderToReadableStream/renderToPipeableStream so React inlines preload hints into headers up to the limit.

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Part of #1328.