Due to the async nature of markdown parsing, sometimes subagent cards
would not scroll fully to the bottom of the content because the
`scroll_to_bottom` function was being called just while the agent was
running... and sometimes, it'd finish running before the markdown had
fully finished parsing. The solution is to remove the `is_running`
conditional.

Release Notes:

- Agent: Improved auto-scroll of subagent content in preview cards,
ensuring the content is always visible until the end.