Markdown Actions: Extending unordered list does not work

### What Happened?

Pressing `` after the last item in an unordered list did not add a new item

### Steps to Reproduce

1. Create an unordered list
2. Note that pressing `` does not add a new item

### Expected Behavior

Unordered lists should extend when `` is pressed on an existing item

### OS Version

7.x (Horus)

### Session Type

Classic Session (X11, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_

未指定源文件、测试或入口点。首先，在 elementary OS 7.x 的 Classic Session 中创建一个无序列表，并在某个列表项后按 Return，以重现该问题。完成的标准是：在现有的无序列表项上按 Return 会创建下一个列表项。

由索引模型根据 Issue 内容生成。