highlighter: Fix markdown inline highlighting across list items (#2535)

### Problem

Markdown inline highlighting could break across list items. Code span
highlighting could stop after the first list item, then apply to later
plain text until another code span appeared.

### Solution

Preserve the separator text between combined markdown inline ranges
before parsing them. This keeps each list item separated during
highlighting. Added a regression test for code spans in markdown list
items.

### Testing

- All highlighter tests (including a new one)
- Manual testing via `cargo run -p gpui-component-story --example
markdown`

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Implemented-by: Codex 5.5
Reviewed-by: Claude Opus 4.8

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Co-authored-by: Jason Lee <huacnlee@gmail.com>