Remove ui dependency from assistant_tool and context_server (#27449)

This PR removes the dependency on the `ui` crate from the
`assistant_tool` and `context_server` crates.

These crates were only depending on it for `IconName`, which can now be
depended on from `icons` directly.

Release Notes:

- N/A

crates/assistant_tool/Cargo.toml

@@ -13,14 +13,14 @@ path = "src/assistant_tool.rs"
 
 [dependencies]
 anyhow.workspace = true
-collections.workspace = true
 clock.workspace = true
+collections.workspace = true
 derive_more.workspace = true
 gpui.workspace = true
+icons.workspace = true
 language.workspace = true
 language_model.workspace = true
 parking_lot.workspace = true
 project.workspace = true
 serde.workspace = true
 serde_json.workspace = true
-ui.workspace = true