Enabled llms.txt leads to markdown being returned in post preview in design preview

[betschki]

Issue Summary

Enabling labs.llmsTxt leads to markdown being returned in the post preview of the design preview:

Assumption:

An Accept: text/plain header is sent to the iframe.

Steps to Reproduce

1. Enable labs.llmsTxt
2. Go to Settings > Design & Branding > Customize
3. Click on the Post tab
4. See issue

Ghost Version

v6.43.1

Node.js Version

22

How did you install Ghost?

Docker

Database type

MySQL 8

Browser & OS version

No response

Relevant log / error output

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