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/assistant_tools/Cargo.toml

@@ -15,9 +15,9 @@ path = "src/assistant_tools.rs"
 eval = []
 
 [dependencies]
+agent_settings.workspace = true
 aho-corasick.workspace = true
 anyhow.workspace = true
-assistant_settings.workspace = true
 assistant_tool.workspace = true
 buffer_diff.workspace = true
 chrono.workspace = true