Fix regression in code blocks in the Content Editor

Fixes a bug where typing a code block input rule was
creating a frontmatter code block instead.

Changelog: fixed

app/assets/javascripts/content_editor/extensions/frontmatter.js

@@ -17,4 +17,7 @@ export default CodeBlockHighlight.extend({
   addNodeView() {
     return new VueNodeViewRenderer(FrontmatterWrapper);
   },
+  addInputRules() {
+    return [];
+  },
 });

spec/frontend/content_editor/extensions/frontmatter_spec.js

+import Frontmatter from '~/content_editor/extensions/frontmatter';
+import { createTestEditor, createDocBuilder, triggerNodeInputRule } from '../test_utils';
+
+describe('content_editor/extensions/frontmatter', () => {
+  let tiptapEditor;
+  let doc;
+  let p;
+
+  beforeEach(() => {
+    tiptapEditor = createTestEditor({ extensions: [Frontmatter] });
+
+    ({
+      builders: { doc, p },
+    } = createDocBuilder({
+      tiptapEditor,
+      names: {
+        frontmatter: { nodeType: Frontmatter.name },
+      },
+    }));
+  });
+
+  it('does not insert a frontmatter block when executing code block input rule', () => {
+    const expectedDoc = doc(p(''));
+    const inputRuleText = '``` ';
+
+    triggerNodeInputRule({ tiptapEditor, inputRuleText });
+
+    expect(tiptapEditor.getJSON()).toEqual(expectedDoc.toJSON());
+  });
+});