[a11y] Fix Title announcement, image flyout dashboards (#220027)

## Summary

This PR resolves [[Platform:Dashboards:DashboardEditMode] Add image
panel is announced
incorrectly](https://github.com/elastic/kibana/issues/215483) issue.
Tested in Safari (Chrome still doesn't pronounce it correctly, which is
a known issue confirmed by Eui team.)

<img width="915" alt="Screenshot 2025-05-04 at 14 06 37"
src="https://github.com/user-attachments/assets/1c2fc8ae-1efc-49b7-ad0b-57eea4cf416c"
/>

src/platform/plugins/private/image_embeddable/public/components/image_editor/image_editor_flyout.tsx

@@ -122,7 +122,7 @@ export function ImageEditorFlyout(props: ImageEditorFlyoutProps) {
     <>
       <EuiFlyoutHeader hasBorder={true}>
         <EuiTitle size="s">
-          <h2>
+          <h2 id="image-editor-flyout-title">
             {isEditing ? (
               <FormattedMessage
                 id="imageEmbeddable.imageEditor.editImagetitle"

src/platform/plugins/private/image_embeddable/public/components/image_editor/open_image_editor.tsx

@@ -84,6 +84,7 @@ export const openImageEditor = async ({
         paddingSize: 'm',
         ownFocus: true,
         'data-test-subj': 'createImageEmbeddableFlyout',
+        'aria-labelledby': 'image-editor-flyout-title',
       }
     );