Currently, when the agent updates its plan, there are a few frames where
the text after `Current:` in the plan summary is blank, causing a
flicker. This is because we treat that field as markdown, and the
`MarkdownElement` renders as blank until the raw text has finished
parsing in the background.

This PR fixes the flicker by changing `Markdown::new_text` to
optimistically render the source as a single `MarkdownEvent::Text` span
until background parsing has finished.

Release Notes:

- N/A

Co-authored-by: Agus Zubiaga <agus@zed.dev>