chore(backend): Fix ClerkRequest helper instance check

[wobsoriano]

Description

Replace instanceof check with duck typing to fix issue where bundlers create multiple Request class instances, causing instanceof ClerkRequest to incorrectly return true for regular Request objects.

This fixes the issue in TanStack React Start (#6996) and other environments where polyfilled or scoped Request classes break instanceof checks.

Builds on top of #7421

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: 82a182b

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

This PR includes changesets to release 11 packages

Name	Type
@clerk/backend	Patch
@clerk/agent-toolkit	Patch
@clerk/astro	Patch
@clerk/express	Patch
@clerk/fastify	Patch
@clerk/nextjs	Patch
@clerk/nuxt	Patch
@clerk/react-router	Patch
@clerk/remix	Patch
@clerk/tanstack-react-start	Patch
@clerk/testing	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

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
clerk-js-sandbox	Ready	Preview	Comment	Dec 10, 2025 8:13pm

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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-clerk-request

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[pkg-pr-new]

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@clerk/agent-toolkit

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@clerk/astro

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@clerk/backend

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@clerk/chrome-extension

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@clerk/clerk-js

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@clerk/dev-cli

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@clerk/elements

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@clerk/clerk-expo

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@clerk/expo-passkeys

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@clerk/express

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@clerk/fastify

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@clerk/localizations

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@clerk/nextjs

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@clerk/nuxt

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@clerk/clerk-react

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@clerk/react-router

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@clerk/remix

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@clerk/shared

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@clerk/tanstack-react-start

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@clerk/testing

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@clerk/themes

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@clerk/types

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@clerk/upgrade

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@clerk/vue

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commit: 82a182b

[jacekradko]

looks good