Agents window Markdown preview renders link reference definitions instead of hiding them

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.137.0
- OS Version: Windows _NT x64 10.0.26200
- Remote: WSL: Ubuntu-24.04
- OS: Linux x64 6.18.33.2-microsoft-standard-WSL2

### Description

The Markdown preview in the Agents window does not consume
[link reference definitions](https://spec.commonmark.org/0.31.2/#link-reference-definitions).
The references themselves resolve correctly and render as links, but every
definition line is additionally shown as a visible block at the bottom of the
document.

Per CommonMark, a link reference definition produces no output of its own.

### Steps to Reproduce

1. Create a file `README.md` with the following content:

```markdown
# Step Templates

| Template | Docs |
| --- | --- |
| `docker-login.yml` | [docker-login.md][1] |
| `docker-logout.yml` | [docker-logout.md][2] |

[1]: docker-login.md
[2]: docker-logout.md
```

2. Open the file in the Agents window.
3. Look at the area below the table.

### Expected behavior

The two definition lines are consumed while resolving `[1]` and `[2]` and
produce no visible output, as they do in the regular VS Code Markdown preview,
on GitHub and in Azure DevOps Repos.

### Actual behavior

Each definition is rendered as a separate code-style block:

```
[1]: docker-login.md
[2]: docker-logout.md
```

See the attached screenshot.

### Additional context

The same file renders as expected in the built-in Markdown preview of the main
VS Code window, so this looks specific to the Markdown renderer used in the
Agents window.