Fix transcript section heading glued to interrupted agent message

[liaowang11]

An interrupted agent_message_chunk leaves the transcript body without
a trailing newline, so the next prompt's heading lands on the same line
as the partial message, e.g.

Actually, I should## User (2026-06-20 19:45:42)

Append a blank-line separator at turn end whenever the last streamed
entry was an agent_message_chunk. Centralize this in a helper called on
both the success and failure (interrupt) paths of the session/prompt
request.

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