Rename `assistant_settings` to `agent_settings` (#31513)

This PR renames the `assistant_settings` crate to `agent_settings`, as
well a number of constructs within it.

Release Notes:

- N/A
8faeb34367Marshall Bowers committed on 5/27/2025, 3:16:55 PM· committed by GitHubparent61a40e2
47 files changedLine totals unavailable