close AI-190
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Introduced an endpoint to retrieve a document's markdown content and
title.
- Added backend support for parsing document snapshots directly into
markdown format.
- **Tests**
- Added comprehensive tests and snapshot files for markdown retrieval,
including success and error scenarios.
- Improved test coverage for content type validation and markdown
parsing utilities.
- **Documentation**
- Enhanced internal documentation through detailed test cases and
snapshot references for new markdown features.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
#### PR Dependency Tree
* **PR #12812** 👈
* **PR #12846**
* **PR #12811**
This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)
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