feat: Add custom domain to article URL if custom domain exists for the portal (#11349)

Portals can have custom domains. When inserting or previewing articles,
we consider the frontend URL. This PR fixes article URL generation to
properly include the portal's custom domain.

app/javascript/dashboard/helper/specs/portalHelper.spec.js

@@ -25,5 +25,47 @@ describe('PortalHelper', () => {
       ).toEqual('https://help.chatwoot.com/hc/handbook/articles/article-slug');
       window.chatwootConfig = {};
     });
+
+    it('returns the correct url with custom domain', () => {
+      window.chatwootConfig = {
+        hostURL: 'https://app.chatwoot.com',
+        helpCenterURL: 'https://help.chatwoot.com',
+      };
+      expect(
+        buildPortalArticleURL(
+          'handbook',
+          'culture',
+          'fr',
+          'article-slug',
+          'custom-domain.dev'
+        )
+      ).toEqual('https://custom-domain.dev/handbook/articles/article-slug');
+    });
+
+    it('handles https in custom domain correctly', () => {
+      window.chatwootConfig = {
+        hostURL: 'https://app.chatwoot.com',
+        helpCenterURL: 'https://help.chatwoot.com',
+      };
+      expect(
+        buildPortalArticleURL(
+          'handbook',
+          'culture',
+          'fr',
+          'article-slug',
+          'https://custom-domain.dev'
+        )
+      ).toEqual('https://custom-domain.dev/handbook/articles/article-slug');
+    });
+
+    it('uses hostURL when helpCenterURL is not available', () => {
+      window.chatwootConfig = {
+        hostURL: 'https://app.chatwoot.com',
+        helpCenterURL: '',
+      };
+      expect(
+        buildPortalArticleURL('handbook', 'culture', 'fr', 'article-slug')
+      ).toEqual('https://app.chatwoot.com/hc/handbook/articles/article-slug');
+    });
   });
 });