Update storybook and tests

stories/ContentBanner.stories.js

@@ -8,6 +8,13 @@ const mockBlogPost = {
       "This is a sample article description that explains what the article covers.",
     author: "Sample Author",
     date: "2025-01-15",
+    thumbnail: {
+      horizontal: "resolving-active-conflicts-horizontal.svg",
+      vertical: "resolving-active-conflicts-vertical.svg",
+    },
+    banner: {
+      horizontal: "resolving-active-conflicts-banner.svg",
+    },
   },
   htmlContent:
     "<p>This is the main content of the sample article.</p><p>It has multiple paragraphs.</p>",
@@ -20,7 +27,7 @@ export default {
     docs: {
       description: {
         component:
-          "The ContentBanner component displays the header information for blog articles, including title, description, author, and date. Note: page background colors are applied at the blog page level using a hex color from frontmatter (background.color), not inside this component. Thumbnail images should be uploaded via the content pipeline to public/content/blog/ and referenced in frontmatter (thumbnail.horizontal / thumbnail.vertical).",
+          "The ContentBanner component displays the header information for blog articles, including title, description, author, and date.\n\nImages: sm uses thumbnail.horizontal; md+ uses banner.horizontal when provided, otherwise falls back to thumbnail.horizontal; final fallback is assets/Content_Banner_2.svg.\n\nNote: page background colors are applied at the blog page level using a hex color from frontmatter (background.color), not inside this component. Thumbnail and banner images should be uploaded via the content pipeline to public/content/blog/ and referenced in frontmatter.",
       },
     },
   },
@@ -38,6 +45,31 @@ export const Default = {
   },
 };
 
+export const NoBannerFallbackToThumbnail = {
+  args: {
+    post: {
+      ...mockBlogPost,
+      frontmatter: {
+        ...mockBlogPost.frontmatter,
+        banner: undefined,
+      },
+    },
+  },
+};
+
+export const NoImagesFallbackToDefault = {
+  args: {
+    post: {
+      ...mockBlogPost,
+      frontmatter: {
+        ...mockBlogPost.frontmatter,
+        thumbnail: undefined,
+        banner: undefined,
+      },
+    },
+  },
+};
+
 export const LongTitle = {
   args: {
     post: {