Let every agent with datasets show them as dataset chips in chat

Any agent that works with datasets can now link them right in the chat. When an agent refers to one of its datasets, the name shows up as a chip you can click to open that dataset. When it suggests several, they appear as a list with a short note under each one saying why it fits.

Before this, only a few built-in assistants did this. It now works for every agent that has datasets attached or can look datasets up, so you can go from an agent's answer straight to the data behind it without searching for it.