agent: Rename a number of constructs from Assistant to Agent (#30196)

This PR renames a number of constructs in the `agent` crate from the
"Assistant" terminology to "Agent".

Not comprehensive, but it's a start.

Release Notes:

- N/A
6cc6e4d4b3Marshall Bowers committed on 5/8/2025, 1:18:51 AM· committed by GitHubparentd6c7cdd
32 files changedLine totals unavailable