language_model: Remove dependency on ui (#27448)

This PR removes the dependency on the `ui` crate from the
`language_model` crate.

We were only depending on it to import `IconName`—which now lives in
`icons`—and some re-exported GPUI items.

Release Notes:

- N/A

Cargo.lock

@@ -7311,6 +7311,7 @@ dependencies = [
  "google_ai",
  "gpui",
  "http_client",
+ "icons",
  "image",
  "log",
  "open_ai",
@@ -7323,7 +7324,6 @@ dependencies = [
  "strum",
  "telemetry_events",
  "thiserror 2.0.12",
- "ui",
  "util",
 ]