Client: enable markdown rendering (#804)

It integrates @a2ui/markdown-it as the markdown renderer for the contact
sample.

This resolves issues where markdown sytax (sch as `##` and `###` header)
was displayed as plain text and ensures that links and button labels are
correctly rendered as shown in the updated UI.

Author: nan-yu

samples/client/angular/projects/contact/src/app/app.config.ts

@@ -14,7 +14,8 @@
  limitations under the License.
  */
 
-import { DEFAULT_CATALOG, provideA2UI } from '@a2ui/angular';
+import { DEFAULT_CATALOG, provideA2UI, provideMarkdownRenderer } from '@a2ui/angular';
+import { renderMarkdown } from '@a2ui/markdown-it';
 import { IMAGE_CONFIG } from '@angular/common';
 import {
   ApplicationConfig,
@@ -33,6 +34,7 @@ export const appConfig: ApplicationConfig = {
       catalog: DEFAULT_CATALOG,
       theme: theme,
     }),
+    provideMarkdownRenderer(renderMarkdown),
     {
       provide: IMAGE_CONFIG,
       useValue: {