Agents window Markdown editor: Revealing Markdown markers shifts following content

Testing #326540

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

- VS Code Version: Insiders with the experimental Markdown editor
- OS Version: macOS
- Audited package revision: microsoft/vscode-packages@1d81418d61ad769f154579196a51a9364c105e2d

### Steps to Reproduce

1. Open a narrow Agents window with this paragraph followed by another block:

```markdown
**This bold sentence is deliberately close to the wrapping boundary**

After
```

2. Leave the bold paragraph inactive so its markers are hidden.
3. Click into the bold paragraph to reveal its markers.

### Expected

Activating the paragraph does not change its line count or move following content. At minimum, active and inactive layouts reserve the same vertical height.

### Actual

At a tested narrow viewport, the paragraph height changed from 36px to 55px and the following paragraph moved down by 19px because the revealed markers caused another wrap.

### Impact

Content moves under the pointer as the user clicks, which can produce misclicks and makes the caret appear unstable. Repeated activation also causes visual jitter and can disturb scroll position in longer documents.