Let users cite text selected in rendered HTML visuals

[saibotma]

Summary

In an assistant message, you can select text and cite it into the composer, and that works in Markdown tables too. It doesn't work in rendered HTML visuals, such as the tables, charts or diagrams an agent publishes as an HTML page. Selecting text there never brings up the cite toolbar. Agents tend to put their most important structured results (comparison tables, summaries) in exactly these visuals, so those are the parts users most often want to quote back.

Steps to reproduce

1. Ask an agent for a result that it publishes as an HTML visual, for example a table comparing student counts per school over several months.
2. Select text inside a table cell in the rendered visual.
3. Expected: the cite toolbar appears, the same as for text in the assistant message.
4. Actual: no toolbar appears. The text can be selected and copied, but not cited.

For comparison, a Markdown table in the assistant message text supports citing as expected.