fix(api): fix unexpected handling of list continuation when the split content is a todo

[bngarren]

Bug: when performing list continuation on the following line, where cursor is |
- □ One|- □Two

It resulted in:

- □ One
- □ - □ Two

Because it didn't recognize the split content being a valid todo line.

With this fix: the above will now result in:

- □ One
- □ Two