ui: copy the displayed text of grouped agentic responses (#27832)
* ui: copy the displayed text of grouped agentic responses
Agentic sessions render as a single entry anchored on the first
assistant turn, whose content is typically just the first tool call,
so the copy button wrote an empty string to the clipboard. Derive the
text sections of the whole session and copy them joined, matching the
visible response. Plain messages keep the previous behavior.
* const