update /extensions to new slash command arch (#4229)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/ui/commands/extensionsCommand.ts

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { type CommandContext, type SlashCommand } from './types.js';
+import { MessageType } from '../types.js';
+
+export const extensionsCommand: SlashCommand = {
+  name: 'extensions',
+  description: 'list active extensions',
+  action: async (context: CommandContext): Promise<void> => {
+    const activeExtensions = context.services.config?.getActiveExtensions();
+    if (!activeExtensions || activeExtensions.length === 0) {
+      context.ui.addItem(
+        {
+          type: MessageType.INFO,
+          text: 'No active extensions.',
+        },
+        Date.now(),
+      );
+      return;
+    }
+
+    const extensionLines = activeExtensions.map(
+      (ext) => `  - \u001b[36m${ext.name} (v${ext.version})\u001b[0m`,
+    );
+    const message = `Active extensions:\n\n${extensionLines.join('\n')}\n`;
+
+    context.ui.addItem(
+      {
+        type: MessageType.INFO,
+        text: message,
+      },
+      Date.now(),
+    );
+  },
+};