Agents window Markdown editor: Select All does not replace a pending paragraph

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 Markdown document containing `Hello`.
2. Put the caret after `Hello`.
3. Press Enter. A new empty paragraph appears.
4. Press Cmd+A.
5. Type `X`.

### Expected

The selected document is replaced with:

```text
X
```

### Actual

The selection appears to cover the document, but typing materializes the pending paragraph instead:

```text
Hello

X
```

### Impact

The visible selection and the edit target disagree. A user performing a destructive replacement can unexpectedly preserve old content and insert somewhere else, which undermines selection fidelity and can produce incorrect documents without an obvious explanation.