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

crates/eval/src/examples/code_block_citations.rs

@@ -1,5 +1,5 @@
+use agent_settings::AgentProfileId;
 use anyhow::Result;
-use assistant_settings::AgentProfileId;
 use async_trait::async_trait;
 use markdown::PathWithRange;