fix: prevent AppShellRegistry validation failure for StyleSheet.Container (#22630)

When AppShellRegistry validates offending annotation on other compoent classes
it ignore the StyleSheet annotation. However, if a component has multiple
StyleSheet annotations on it, the validation fails.
This change also ignores StyleSheet.Container annotation during validation

Fixes #22629

Author: mcollovati

flow-server/src/main/java/com/vaadin/flow/server/AppShellRegistry.java

@@ -176,6 +176,7 @@ public String validateClass(Class<?> clz) {
         // StyleSheet is allowed on Components; do not treat it as an
         // AppShell-only annotation
         validOnlyForAppShell.remove(StyleSheet.class);
+        validOnlyForAppShell.remove(StyleSheet.Container.class);
         if (WebComponentExporter.class.isAssignableFrom(clz)) {
             // Webcomponent exporter should have the theme annotation
             // and Push annotation as it is not appShell configured.

flow-server/src/test/java/com/vaadin/flow/server/startup/VaadinAppShellInitializerTest.java

@@ -208,6 +208,13 @@ public static class AppShellExtendingComponent extends Component
     public static class ComponentWithStylesheet extends Component {
     }
 
+    @StyleSheet("./foo1.css")
+    @StyleSheet("./foo2.css")
+    @StyleSheet("./foo3.css")
+    @StyleSheet("./foo4.css")
+    public static class ComponentWithMultipleStylesheet extends Component {
+    }
+
     @StyleSheet("context://my-styles.css")
     @StyleSheet("https://cdn.example.com/ui.css")
     public static class MyAppShellWithStyleSheets
@@ -554,6 +561,13 @@ public void styleSheetOnComponent_notOffending() throws Exception {
         initializer.process(classes, servletContext);
     }
 
+    @Test
+    public void multipleStyleSheetOnComponent_notOffending() throws Exception {
+        classes.add(ComponentWithMultipleStylesheet.class);
+        // Should not throw as @StyleSheet is allowed on Components
+        initializer.process(classes, servletContext);
+    }
+
     @Test
     public void styleSheetResolution_variousScenarios() throws Exception {
         classes.add(MyAppShellWithVariousStyleSheets.class);