fix(nextjs): Comply with Server Actions async requirement

[wobsoriano]

Description

This PR fixes a build error that occurs when using Next.js 16.1.0-canary.19 (which includes security fixes for CVE-2025-55183 and CVE-2025-55184).

Build error (without async):

▲ Next.js 16.1.0-canary.19 (Turbopack)
- Environments: .env

  Creating an optimized production build ...

> Build error occurred
Error: Turbopack build failed with 1 errors:
./node_modules/@clerk/nextjs/dist/esm/server/keyless-custom-headers.js:70:10
Ecmascript file had an error
  68 |   }
  69 | }
> 70 | function formatMetadataHeaders(metadata) {
     |          ^^^^^^^^^^^^^^^^^^^^^
  71 |   const headers2 = new Headers();
  72 |   if (metadata.nodeVersion) {
  73 |     headers2.set("Clerk-Node-Version", metadata.nodeVersion);

Server Actions must be async functions.

Import trace:
  Server Component:
    ./node_modules/@clerk/nextjs/dist/esm/server/keyless-custom-headers.js
    ./node_modules/@clerk/nextjs/dist/esm/app-router/server/keyless-provider.js
    ./node_modules/@clerk/nextjs/dist/esm/app-router/server/ClerkProvider.js
    ./node_modules/@clerk/nextjs/dist/esm/index.js
    ./app/layout.tsx


    at ./node_modules/ (clerk/nextjs/dist/esm/server/keyless-custom-headers.js:70:10)

Build success (with async):

▲ Next.js 16.1.0-canary.19 (Turbopack)
- Environments: .env

  Creating an optimized production build ...
✓ Compiled successfully in 2.1s
✓ Finished TypeScript in 1818.7ms
✓ Collecting page data using 9 workers in 441.3ms
✓ Generating static pages using 9 workers (8/8) in 93.0ms
✓ Finalizing page optimization in 11.6ms

formatMetadataHeaders is inside of a 'use server' file.

Discord ticket: https://discord.com/channels/856971667393609759/1449025014752874516

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

[changeset-bot]

🦋 Changeset detected

Latest commit: 36ca0d5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@clerk/nextjs	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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Project	Deployment	Preview	Comments	Updated (UTC)
clerk-js-sandbox	Ready	Preview	Comment	Dec 12, 2025 5:58pm

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch rob/fix-keyless-server-function

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commit: 36ca0d5