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/collab/src/tests/test_server.rs

@@ -312,7 +312,7 @@ impl TestServer {
             );
             language_model::LanguageModelRegistry::test(cx);
             assistant_context_editor::init(client.clone(), cx);
-            assistant_settings::init(cx);
+            agent_settings::init(cx);
         });
 
         client