markdown_preview: Fix markdown tasklist not being shown

[AdamJedl]

Fixes a regression introduced in #52008

It's because pulldown-cmark may insert Paragraph between Item and TaskListMarker

example markdown:

- [x] a

- [ ] b

before:

after:

ParsedMarkdown

ParsedMarkdown {
    source: "- [x] a\n\n- [ ] b\n",
    events: [
        (
            0..17,
            RootStart,
        ),
        (
            0..17,
            Start(
                List(
                    None,
                ),
            ),
        ),
        (
            0..9,
            Start(
                Item,
            ),
        ),
        (
            6..8,
            Start(
                Paragraph,
            ),
        ),
        (
            2..5,
            TaskListMarker(
                true,
            ),
        ),
        (
            6..7,
            Text,
        ),
        (
            6..8,
            End(
                Paragraph,
            ),
        ),
        (
            0..9,
            End(
                Item,
            ),
        ),
        (
            9..17,
            Start(
                Item,
            ),
        ),
        (
            15..17,
            Start(
                Paragraph,
            ),
        ),
        (
            11..14,
            TaskListMarker(
                false,
            ),
        ),
        (
            15..16,
            Text,
        ),
        (
            15..17,
            End(
                Paragraph,
            ),
        ),
        (
            9..17,
             q
        (
            0..17,
            End(
                List(
                    false,
                ),
            ),
        ),
        (
            0..17,
            RootEnd(
                0,
            ),
        ),
    ],
    languages_by_name: {},
    languages_by_path: {},
    root_block_starts: [
        0,
    ],
    html_blocks: {},
    mermaid_diagrams: {},
}

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed markdown tasklist not being shown