Add example plugin

crates/egui_kittest/examples/require_button_labels_plugin.rs

@@ -0,0 +1,50 @@
+//! Example that shows how to create a `egui_kittest` Plugin that ensures each button has a label.
+
+use egui_kittest::kittest::{NodeT as _, Queryable as _};
+use egui_kittest::{Harness, Plugin};
+
+/// Plugin that panics if any visible button in the current UI lacks a non-empty label.
+pub struct RequireButtonLabels;
+
+impl<S> Plugin<S> for RequireButtonLabels {
+    fn after_step(&mut self, harness: &mut Harness<'_, S>) {
+        for button in harness.query_all_by_role(egui::accesskit::Role::Button) {
+            let node = button.accesskit_node();
+
+            match node.label().as_deref() {
+                Some(label) if !label.is_empty() => {}
+                _ => panic!(
+                    "Button at {:?} has no accessible label. \
+                     Every button must be labelled so screen readers and kittest queries \
+                     can address it.",
+                    node.bounding_box(),
+                ),
+            }
+        }
+    }
+}
+
+fn main() {
+    // Check tests below for usages
+}
+
+#[test]
+fn test_has_label() {
+    let mut harness = Harness::builder()
+        .with_plugin(RequireButtonLabels)
+        .build_ui(|ui| {
+            let _ = ui.button("Test");
+        });
+    harness.run(); // this is fine
+}
+
+#[test]
+#[should_panic]
+fn test_no_label() {
+    let mut harness = Harness::builder()
+        .with_plugin(RequireButtonLabels)
+        .build_ui(|ui| {
+            let _ = ui.button(());
+        });
+    harness.run(); // BOOM
+}