gpui: Rename rounded_md to rounded_sm (#26228)

This PR renames the `rounded_md` style method to `rounded_sm`.

Follow up to https://github.com/zed-industries/zed/pull/26221, which
freed up the `rounded_sm` name.

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -645,7 +645,7 @@ impl ActiveThread {
             Role::System => div().id(("message-container", ix)).py_1().px_2().child(
                 v_flex()
                     .bg(colors.editor_background)
-                    .rounded_md()
+                    .rounded_sm()
                     .child(message_content),
             ),
         };