Rendered Markdown in Source Control injects diff markup into Mermaid code blocks, causing parse errors

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

* VS Code Version: 1.134.0 (Universal)
* OS Version: macOS Tahoe 26.5.2

Steps to Reproduce:

1. Create a Markdown file containing:

````markdown
# Mermaid diff test

```mermaid
erDiagram
USER ||--o{ TEAM: "have"
USER ||--o{ TEAM_MEMBER : "belongs to"
```
````

2. Commit the file to Git.
3. Delete `USER ||--o{ TEAM_MEMBER : "belongs to"`
4. Open the changed Markdown file from the Source Control view.
5. Switch the diff editor from **Text Diff Editor** to **Markdown Preview**.
6. Mermaid diagram fails to render with a parse error.

```text
Parse error on line 3:
...||--o{ TEAM: "have"

Rendered Markdown diff: Mermaid reports a parse error containing `` markup