Preserve TUI markdown list spacing after code blocks

[etraut-openai]

Why

Fixes #19702.

The TUI markdown renderer could visually attach the next list marker to a fenced code block inside the previous list item, even when the source markdown included a blank line before the next item. That made block-heavy loose lists harder to read, while the desired behavior is still to keep simple lists compact.

What changed

• Track whether the current rendered list item contains a code block.
• Preserve one blank separator before the following list marker only when the previous item contained a code block.
• Add regression coverage for both paths: code-block list items keep the separator, and simple loose list items stay compact.

Verification

• cargo test -p codex-tui markdown_render

I also manually verified that the bug exists before and is fixed after.

Before

After

[fcoury-oai]

I think it's a good idea to use the presented solution to catch other edge cases.