Mark the widgets a DragValue is built from with DragValue::CLASS

A drag value is not a widget of its own: it shows a `Button` while you drag
it and a `TextEdit` while you type into it. A `StyleProvider` asked to style
either had no way to tell it apart from a stand-alone one, so it could not
style the two states as one control.

Set `egui::drag_value` on the drag value itself; it already forwards its
classes to both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/widgets/drag_value.rs

@@ -71,6 +71,9 @@ pub struct DragValue<'a> {
 impl<'a> DragValue<'a> {
     const ATOM_ID: &'static str = "drag_item";
 
+    /// Present on the [`Button`] and the [`TextEdit`] a drag value is built from.
+    pub const CLASS: &'static str = "egui::drag_value";
+
     pub fn new<Num: emath::Numeric>(value: &'a mut Num) -> Self {
         let slf = Self::from_get_set(move |v: Option<f64>| {
             if let Some(v) = v {
@@ -100,7 +103,7 @@ impl<'a> DragValue<'a> {
             custom_formatter: None,
             custom_parser: None,
             update_while_editing: true,
-            classes: Classes::default(),
+            classes: Classes::default().with_class(Self::CLASS),
             min_size: None,
         }
     }