Update Compound's List header style to match our iOS 26 components. (#4797)

* Update Compound's list header font for iOS 26.

* Update the preview snapshots.

* Update UI test snapshots.

compound-ios/Sources/Compound/List/ListStyles.swift

@@ -18,7 +18,7 @@ public extension View {
     
     /// Styles a list section header using the Compound design tokens.
     func compoundListSectionHeader() -> some View {
-        font(.compound.bodySM)
+        font(headerFont)
             .foregroundColor(.compound.textSecondary)
             .listRowInsets(EdgeInsets(top: 15,
                                       leading: ListRowPadding.horizontal,
@@ -35,6 +35,16 @@ public extension View {
                                       bottom: 10,
                                       trailing: ListRowPadding.horizontal))
     }
+    
+    // MARK: - Private
+    
+    private var headerFont: Font {
+        if #available(iOS 26.0, *) {
+            .compound.bodyMDSemibold
+        } else {
+            .compound.bodySM
+        }
+    }
 }
 
 struct ListTextStyles_Previews: PreviewProvider, TestablePreview {