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/ui/context_pill.rs

@@ -103,7 +103,7 @@ impl RenderOnce for ContextPill {
             .pl_1()
             .pb(px(1.))
             .border_1()
-            .rounded_md()
+            .rounded_sm()
             .gap_1()
             .child(self.icon().size(IconSize::XSmall).color(Color::Muted));