markdown: Fix render list item blocks (#2585)

Closes #[issue number]

## Description

Markdown block elements such as fenced code blocks, tables, blockquotes,
and headings were being dropped when nested inside list items. These
blocks now render at the full available list width, with regression
coverage for visibility and code-block action alignment.

## Screenshot

| Before                       | After                       |
| ---------------------------- | --------------------------- |
|<img width="1594" height="533" alt="Screenshot 2026-07-22 at 3 10
36 pm"
src="https://github.com/user-attachments/assets/68558d68-a152-42c1-80a6-fa9e50b7ac59"
/> | <img width="1242" height="511" alt="Screenshot 2026-07-22 at 3 05
54 pm"
src="https://github.com/user-attachments/assets/61023976-bf05-4206-ae10-d5fa74f73641"
/> |




## How to Test

Run `cargo run --example markdown` with this fix applied:
````markdown
# Hello, **World**!
  
1. This is the first list item Hello!
    ```text
    a nested block
    ```
2. second list item
  
```text
non-nested block
```
````
## Checklist

- [ X ] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [ X ] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ X ] Passed `cargo run` for story tests related to the changes.

---------

Co-authored-by: Richard Nelson <rnelson@atlassian.com>
Co-authored-by: Jason Lee <huacnlee@gmail.com>