assistant2: Include the thread summary in the Markdown representation (#26693)

This PR adds the thread's summary (if it has one) as a heading in the
Markdown representation.

Release Notes:

- N/A
1be3f81920Marshall Bowers committed on 3/13/2025, 4:59:35 PM· committed by GitHubparent2eb4d6b
1 file changedLine totals unavailable