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/context_server/Cargo.toml

@@ -21,6 +21,7 @@ context_server_settings.workspace = true
 extension.workspace = true
 futures.workspace = true
 gpui.workspace = true
+icons.workspace = true
 language_model.workspace = true
 log.workspace = true
 parking_lot.workspace = true
@@ -30,6 +31,5 @@ serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
 smol.workspace = true
-ui.workspace = true
 url = { workspace = true, features = ["serde"] }
 util.workspace = true