Docs: Adjust Next.js snippets to include draftmode

[createhb21]

What I changed

• Added draftMode usage to docs/_snippets/nextjs-headers-mock.md for all snippet variants (CSF 3/Next, JS/TS).
• Demonstrated how to mock draft mode state via draftMode.mockReturnValue(...).
• Fixed an incorrect assertion in the same snippet (cookies().get('timezone') -> headers().get('timezone')).

Why

Closes #28851

Verification

• yarn prettier --check docs/_snippets/nextjs-headers-mock.md

Summary by CodeRabbit

• Documentation

  • Updated Next.js headers mocking documentation to include draft mode support alongside existing cookies and headers functionality.

• Tests

  • Enhanced mock setup to verify draft mode handling with proper initialization and assertions.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between b7355d8 and 109f76f.

📒 Files selected for processing (1)

• docs/_snippets/nextjs-headers-mock.md

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📝 Walkthrough

Walkthrough

A documentation snippet for Next.js headers mocking is updated to include draftMode alongside existing cookies and headers functionality, with corresponding mock setup and test assertions added across multiple code variants (CSF 3, TypeScript, and JavaScript versions).

Changes

Cohort / File(s)

Summary

Next.js Headers Mock Documentation docs/_snippets/nextjs-headers-mock.md

Added fn import from Storybook test utilities; extended headers import to include draftMode; introduced draftMode mock setup in beforeEach hook returning isEnabled, enable, and disable functions; updated test assertions to verify draftMode calls alongside timezone checks; revised documentation comments to reference draft mode handling; wired mock initialization across all code variants (CSF 3, TS, and Next.js TS/JS blocks).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• storybookjs/storybook#33380: Wires draftMode into headers mock exports and re-exports alongside the main PR's documentation updates.
• storybookjs/storybook#33341: Changes runtime draftMode resolution in headers mock for v14 compatibility, complementing the mock setup patterns documented in this PR.

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[valentinpalkovic]

LGTM! @jonniebigodes Can you also please take a look?

[jonniebigodes]

@createhb21 appreciate you taking the time to put together this pull request and help us improve the documentation 🙏 ! I've checked, and all is good on my end.

Will gladly merge this once the checklist clears.

Have a great day

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