🐛 Fixed design preview showing raw markdown when llms.txt is enabled

[betschki]

fixes #28265

• the design & branding and announcement bar previews fetch the page, read it as text, and re-parse it as HTML into the iframe. They have always sent Accept: text/plain purely as a "give me the raw body" signal, which the frontend ignored
• the llms.txt feature added Accept-based content negotiation that now treats text/plain as a request for the markdown variant, so with labs.llmsTxt enabled these previews received markdown and rendered it as garbled text
• the previews genuinely want HTML, so they now ask for text/html instead of accidentally opting into markdown content negotiation

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: f0c6a9fd-84e8-46e1-9788-8bc8681540b1

📥 Commits

Reviewing files that changed from the base of the PR and between 033866e and 028f0ad.

📒 Files selected for processing (3)

• apps/admin-x-settings/src/components/settings/site/announcement-bar/announcement-bar-preview.tsx
• apps/admin-x-settings/src/components/settings/site/design-and-branding/theme-preview.tsx
• apps/admin-x-settings/test/acceptance/site/design.test.ts

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Walkthrough

This PR fixes a bug where markdown was being returned in design previews instead of HTML when the llmsTxt feature was enabled. The fix updates the HTTP Accept header from text/plain to text/html in two preview components—announcement bar preview and theme preview—ensuring they request the correct content type. A corresponding acceptance test assertion was added to verify the header is correctly set to text/html.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change - fixing design preview markdown rendering when llms.txt is enabled, which is the primary objective.
Description check	✅ Passed	The description clearly explains the root cause, what was changed, and why - directly addressing the llms.txt content negotiation issue affecting previews.
Linked Issues check	✅ Passed	The PR successfully addresses issue #28265 by changing the Accept header from text/plain to text/html in both preview components and adding corresponding test assertion.
Out of Scope Changes check	✅ Passed	All changes are directly related to the issue objective - updating Accept headers in two preview components and adding test coverage for the fix.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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